Back when I was first in grad school, studying conducting, we would sit around and nerd out to audio and video of different performances by various conductors. It's fun (if you're into that kind of thing), you learn a lot, and you really stimulate your own gestural imagination by analyzing exactly how really effective conductors are able to have the impact they can have. Our favorite at the time, and someone who remains a true titan in field of conducting, was Carlos Kleiber.
A virtuosic conductor, Kleiber married technical precision with a graceful, elastic expressiveness that--when paired with a finely detailed rehearsal process--allowed him to lead large ensembles in truly passionate, spontaneous performances. Video and audio recordings of his performances are few, but recordings of his rehearsals are even fewer. Below is video of Kleiber leading a rehearsal of Die Fledermaus overture, in 1970. The whole rehearsal is very entertaining and well worth watching, but the clip below starts a few minutes in so that I can point out a few specific things about his work.
First, watch this video, which starts at about 6:45, until around the 10:00 mark:
In that clip, Kleiber first seeks to change the way the violins play the melody by using poetic and detailed gesture, imagery, and descriptions of mood. Importantly, as he is verbally describing the way he wants the melody to be played, he is simultaneously expressing what he wants to hear gesturally, showing the kinds of movement he will shortly use when the orchestra plays the passage again. This makes an important and powerful connection to how he will conduct the passage when the orchestra is playing, what his movements mean more generally, and begins the process of teaching the players that not only will he will be asking for very specific things in his conducting but how he will be using his movements to ask for them. And he does all of this without being didactic or pedantic; indeed, he is charming and engaging in his humor and enthusiasm.
Over the next few minutes of the clip, he continues this process, gradually moving the ensemble into more enthusiastic and specific stylistic commitment in their playing, and a more free, flexible sense of tempo. Notice how alive the baton seems in his hand, how expressively precise: that's what technical mastery in conducting looks like.
The next video clip starts during the performance following the rehearsal seen above, at the same point in the overture. The orchestra's playing is lovely, expressive, and beautifully together in a way it simply was not at the start of the rehearsal. Amazingly, all that expression is in Kleiber's gestures, precisely, clearly, and vividly:
For a more focused demonstration of his remarkable work, below is a video of Kleiber leading a performance of the Strauss waltz Die Libelle. It's a master class in conducting all by itself (my favorite moment is the transition from 3:48-4:02, so beautifully led and exquisitely played..and that smile! He was happy about that one.):