In a machine including structure for sensing a sheet fed to the machine, an improvement comprising: a rotary timing cam, an actuating member movable into and out of locking engagement with the cam; apparatus for controlling the actuating member, the controlling apparatus including resilient structure for urging the actuating member to move into and out of locking engagement with the cam; and the sensing structure including structure for normally latching the controlling apparatus for preventing the resilient structure from moving the actuating member out of locking engagement with the cam, and the latching structure unlatching the controlling apparatus for permitting the resilient structure to move the actuating member out of locking engagement with the cam when the sensing structure senses a sheet fed to the machine.