the watch comprises a detecting device including a resilient blade , the free end of which cooperates with a disk . when a slot in the edge of the disk is oriented in the direction of the blade , the latter vibrates owing to its own resiliency . the two edges of the slot are preferably bent in opposite directions in order to increase the bending stress on the blade just before its abrupt release into the slot . a piezoelectric strip fixed to the blade registers voltage fluctuations , the polarity of which corresponds to the direction in which the blade moves .