Realtime group scheduling doesn't know JACK

Actually one of the ways to play it *is* with headphones that pick up what you're hearing and rebroadcast it to you delayed by just enough (a changing delay): then all you have to do is keep what you hear from becoming phased! (Another common way, and probably the more effective one, is to play the second part without a tape of the first part at all, just with a metronome beat defining when the first part's beats are, then mix the two together later. But you can't do that live and it feels obscurely like cheating. Live performers generally have to do it without any artificial assistance at all, and that *is* hard.)