As an amateur musician, having played in different sitiuations, eye contact with the conductor or drum major is important. Professionals can probably do all of

As an amateur musician, having played in different sitiuations, eye contact with the conductor or drum major is important. Professionals can probably do all of this without a conductor on stage but when things start to fall apart and go out of sync, it's kind of a weird feeling. You have to force yourself to go deaf and play the music in time with the conductor's gestures until you're back in sync again. If you don't go deaf you will end up playing in time to the people around, you who may or may not be in sync with the conductor.