Agree with Clive above that the first person in the Uk that i was aware of using the 'foot up' method was John K in the early seventies. It is now universally understood, but is not the only method. A 'hup' works if you play an instrument that leaves your mouth free, or on a loud instrument, a subtle change in the arrangement, e.g. a slowing down, or a extended embellishment, works. Session players are suprisingly good at reading each other's signals.