This seems to come up for each of my aircraft, as it seems that many are unfamiliar with the real world behavior of this system. From the POH:
"When the switch is placed in the AUTO position and released, it will activate one cycle. Each time a cycle is desired, the switch must be pushed to the auto position and released. In the event of a malfunction in the timer, the switch can be held momentarily in the MANUAL position to achieve simultaneous inflation of all the deice boots."
Hope that answers your question! I'm always here if you have any others.