If you beat timeto a piece of music, you move your hand or foot up and down in time with the music.A conductor beats time to show the choir or orchestra how fast they should sing or play the music.
He beats time with hands and feet.
See full dictionary entry for beat
beat time in British English
(of a conductor, etc) to indicate the tempo or pulse of a piece of music by waving a baton or a hand, tapping out the beats, etc