i say, as far as applauding a dancer while he or she is still performing is a matter of personal preference. i don't applaud during a dancer's fouettes merely because i am always silently egging them on. i know how difficult those turns are and am always gripping the edge of my seat watching every detail of the turns! once they finish, i clap until my hands are raw! however, the dancers are up there performing for the audience and probably love to hear their reaction to certain steps as they occur. if the audience is too reserved, then it can be a bit of a letdown to the dancers and they will lose some of their performance energy. it will make them feel as if they haven't engaged their audience. i feel that some of the best performances are done for children because the kids laugh, gasp, boo, scream, etc. whenever they feel like it. that shows they are captivated by what they are watching and the dancers know they are really reaching out to the audience.
also, the corps de ballet loves to get applause. they work for hours trying to match each other, reviewing every detail of the head and arms. so if their formations are neat and they move in unison, please let them know you appreciate it!
