Tempo is perhaps one of the most important aspects of conducting. Shaping the tempo, moving the tempo, and starting the tempo are all incredibly important aspects of every moment in music-making. All of it is a bit like magic. Somehow you have to get 70 + musicians of the orchestra all to get the same idea at the same time about how the tempo will go. Everyone has to move, exactly together, and the conductor has to enable that to happen clearly, succinctly and with authority. Ideally you want to get it right the first time. Orchestras have great memory for tempo. When they feel a tempo, they remember and will go back to this tempo.
This is great because it is something you can mostly count on and build upon in rehearsals and concerts, but it's dangerous to stay on a wrong tempo for this reason when you first begin to rehearse.
In my earlier days of conducting I had trouble with this. If the tempo was sluggish compared to how I wanted it, I didn't know how to change it to exactly what I wanted. It took considerable time and experience to understand the degree of confidence and fortitude required. A conductor needs to get into the right tempo as quickly as possible and rectify anything that isn't right. You need to know exactly how to get in front of the orchestra just enough to bring them with you but not to lose them. You have to be unflinching in showing exactly what the tempo is even if everyone is scampering around trying to guess and second guess it.
Your threshold for chaos has to withstand the moment of confusion if necessary to show that you are clear about the tempo and what you want it to be and to follow through with settling it.
First, know the tempos that you want to go in. Trust me, if you aren't quite sure, it will come across. Second, don't be scared to move ahead or pull back until you get exactly what you want. There can be a tendency to feel like you have to go back and explain what you want differently in the tempo. Don't do it (unless you really have to). Just stick to your guns and move it where it needs to go, or go back and try again without saying anything. But also, realize that sometimes, especially with new works, or with new experiences, you aren't going to get it exactly right at first.
The acoustic of the hall, the tendency of an orchestra (which might be new to you), the orchestra's relationship or lack of relationship with the music, and your own understanding on how the music will sound best with the best tempo is all something that you are going to learn in the moment. You have two options. Gradually move things to where you want them to be. And don't be scared in a second or third read to continue to make adjustments. And certainly, especially if you are confident in knowing what you are doing, don't hesitate to occasionally share your dilemmas or searches for the right tempo with the orchestra at times. It's okay for them to know that you are trying to get something perfect and that you are searching for that perfection. It gives you the chance to share the experience of trying to make great music with them. Everything is in balance.
There are moments when you want to manipulate tempo without saying anything. But sometimes it is good to share with musicians too that you, like them, are exploring, working hard to find the right answer, searching yourself.
Getting the exact tempo is sometimes more than just about speed. It is about the feeling between the notes, and the direction of the phrasing. My experience is that there is a very specific line between conducting the quarter note if it is a fast tempo, and conducting either the half note or the bar if it is possible (fast enough) instead. You have a choice. Do you conduct fast beats with the orchestra or do you conduct slower beats with the fast notes falling between the beats' Often, using the half bar or conducting in bigger beats is a great way to get an orchestra to move. An orchestra is a big machine'.if you want to get it to move, compel it through big beats. It can read this better because then you are more flowing over the beats than right down there in the trenches 'with' the beats. You can use this gesture to move things, to get out of the smaller beat which is strongly being felt by the orchestra and they are holding to, and creating the flow and forward momentum that you want by conducting a bigger beat. If you want an accelerando for instance, go into cut time (if it's in four) or into the full bar (if in three). Likewise, when you need to gear into a slow down, you can flip from conducting the bigger slower beats into faster beats (from the half bar then to quarter notes for instance) which gives you more control and then you can slow down. The more flow you want: the slower your beats and the more energy between those beats. The slower you want to go: the more you beat, the more control you have.
Flow is extremely important with music. You have to have enough flow in your arms for everyone to feel where the next beat is going to be. If your arms stop, everyone is guessing. In extremely slow music, there are two options. If you need to have control, you will subdivide wherever you need to have the flow and control in good balance. However, with experience you will also know that in slow music, the musicians will create the flow themselves. You have to trust that the space between the beats will be felt rather than shown. This is actually very liberating when you understand that just by listening then, and letting the flow or direction of the sound become the unifying and directional force, that your hands become more subliminal. Now you just guide the 'edges' of the sound, the moments of change. Or you flow with the overall phrase and slightly mould it, without actually defining the movement itself. In these moments everyone understands that the musicians lead a lot of the motion. Ears become more important, and eyes less.
Flowing gestures in the key moments are important here. You follow the sound and manipulate with clear gestures where you need to.
Sometimes the music requires us to be shocking and sudden. A beat or two (in the new tempo) in advance can be a good idea in this situation. Faster beats at least to start are mandatory in this situation. You can always flip into a bigger beat once the flow and tempo is established. I totally believe in becoming small in the beat or somewhat subliminal when the orchestra is running with a tempo. Take your hands out of the mix and let them play.
In a first read, orchestras will often tend to play under the tempo that you might want at the starting of music if you aren't extremely clear and persuasive. You have to compel them to exactly the right tempo and once it is clear they will keep it. Use the big beats to your advantage whenever a tempo isn't working as fast as you want it too, especially on first reads. You can always go back to smaller beats once the tempo is established.
Rushing is a separate issue. Here you have to make sure that you help the orchestra define whether they are playing vertically on the middle of the beats, playing heavy (just on the backside or keeping it firmly weighted), or whether they are flowing a bit onto the frontside of the beat. This aspect changes from phrase to phrase depending on the character of the music. Once things start to flow to the front side of the beat, you get potential for rushing. There are many moments where you want this to happen in order to enable a phrase to musically shape itself forward. Playing on the front side of the beat is a great technique but has to be controlled. Usually these moments are short-lived, are specific to a phrase or a certain drive to a newer faster tempo. If a larger section starts to live on the front side of the beat, you get sloppiness, a scampering sort of sound and the potential for mistakes. Tempo is critical.
Recently I conducted two works by Copland: Appalachian Spring and the Clarinet concerto. In both of these works, there were key situations where one had to make sure to keep the tempo 'held' to ensure absolute success. In the train section of the Appalachian Spring, the tempo wants to rush, and it is your job to select the right tempo for the group you are working with and to insist on vertical placement, especially of the downbeats to keep everything clean. In the Copland Clarinet Concerto, I also found that there is an enormous difference in performance success between two tempos that might be but a mere degree apart. One tempo too fast at the Allegro section, and you get chaos. One tempo less, creates a determined and placed sound. You have to determine in the music when it is worth the wildness and risk, and when it is right to have everything controlled.
In the same concert I used the technique of going into the big beats to great advantage while conducting Gershwin's Rhapsody in Blue. Here I was able to use the technique of moving into two beats to the bar (rather than conducting four quarter notes to the bar) and had complete control in driving the orchestra forward to a very fast tempo using this technique. In order to do this you have to have contact and communication with the driving inner rhythm and make it clear to them that you are specifically moving THEM! Especially because this music is well known, there was a great flexibility in the orchestra which could be manipulated. One should push the boundaries and have fun doing so in such a situation.