I recently watched a recording of Jean Michel Jarre's Twelve Dreams of the Sun concert and noticed that his drummer seems to rhythmically move to a beat not audible to the audience, e.g. as 11:10. You can clearly see he's having fun doing what he does, but what exactly might he be listening to?
He is probably listening to a click track, which is a kind of sophisticated metronome. In concerts like this, most of the music is being played by live musicians, but some parts of the music you are hearing have been pre-recorded in a recording studio. With each piece of music, the drummer is listening to a rhythmic clicking sound which is synchronized to the beat of the pre-recorded parts. This enables the entire band to perform their live music alongside the pre-recorded tracks and to keep everything tightly synchronized.
I skimmed through your youtube video, and found a spot at about 1:07:08 or 1:07:09 that I think shows what you're talking about. We hear strings and piano, the drummer has some rests, nods his head with the beat and then comes in. In this case my guess is he isn't listening to a click track, because there is a conductor -- I saw the conductor at, for example, 1:03:19. Now, if the conductor had headphones too, then I could see that they both might be listening to a recording of the piece.
I think that at the 1:07:08 spot, the drummer is probably moving his head to the beat, feeling his rests before he comes in.
If you're wondering why the drummer is wearing headphones at all, I believe it's to protect his hearing. He needs to hear (monitor) what the rest of the ensemble is doing; and he needs to hear himself but with reduced volume.
Personally, I listen to an audiobook of the latest Stephen King whilst playing a gig on the drums.
More seriously, there's three likely answers:
- A click track, basically a metronome pulse, possibly with the same signal being heard by some of the other musicians, keeping them in time.
- A Monitor track, they're hearing a mix of the music which is taylored to the drummer, it'll probably have a lot of the bass sounds and vocals.
- Far more rarely, there may be instructions from a musical director, including some real-time notes on the performance. This doesn't happen often unless it's a musical theater-type performance, and would often be heard as well as one of the above signals.
He may be hearing a click or other pre-recorded material - a simple sequenced drum track can be more useful to play with than a simple click. He may be monitoring the rest of the band - wedge monitors are not always appropriate among the forest of mics often used on a drum kit. Or, if he's sensible and wants to have some hearing left when he's 40, they might be ear protectors.
My drummer for a school production once was very attached to his ear protectors. Very sensible too, when he was playing in his rock band. It took some time to persuade him that in THIS situation his job was to play quieter, so he didn't NEED the protectors.
An additional consideration which I don't think has been mentioned yet.
In huge stadium production gigs such as those put on by Jean-Michel Jarre there are other elements than just the music, such as video projections, pyrotechnics, etc.
The drummer will be listening to a click track and/or other cues in their headphones so that they, and as a consequence the rest of the band, are precisely in time with the computer system that is driving the non-musical effects. Any slight timing errors between the band and the video wall behind them would be glaringly obvious, so the drummer follows the click track to make sure that doesn't happen.