A valve for causing pulsatile flow in a multi-piston plunger pump includes a valve poppet, a valve seat, and a spring biasing the valve poppet against the valve seat; and a mechanism holding the valve poppet in an open position, thereby producing pulsatile flow from the pump. The mechanism includes a magnet and an adjustment mechanism for moving the magnet toward and away from the valve poppet. When the magnet is moved toward the valve poppet, the magnet attracts the poppet away from the valve seat, and when the magnet is moved away from the valve poppet, the magnet releases the poppet. The spring then biases the poppet against the valve seat in a closed position.