An apparatus for seamlessly activating an on-demand function related to a fluid-driven instrument connects to an output of control valve regulating system pressure during normal operation of the instrument. The apparatus includes a fluid-operated bistable circuit that is switchable from a standby mode to an actuating mode by a brief drop in system pressure. In the actuating mode the circuit acts on a switch or valve to activate the function. Turning off system pressure to a longer time returns the circuit to its standby mode. The circuit includes a two-port biased actuator responsive to pressure imbalance between its ports, which are separately pressurizable through, respectively, an actuator-controlled valve and a flow control module.