One of a series of devices for improvisational practice, invented by Phish. For an example of one variation, each band member in a session locks onto a particular groove, then one changes what they're doing slightly, and the others all try to adjust; when they do, the changer says "hey", and the next person makes a change.

"If you're not happy every morning when you get up, leave for work, or start to work at home--if you're not enthusiastic about doing that, you're not going to be successful."
-- Donald M. Kendall