If you are very observant you might notice it draining and filling during the wash cycle itself. This was a mystery to me until I read the service docs.
The APF (Automatic Purge Filtration) wash system allows the control to continuously filter and flush
food soil out of the pump during “APF enabled” intervals scattered throughout each cycle and do it
without interrupting the cycle. The control monitors the pressure switch (soil sensor) input during each
of the APF enabled intervals in the cycle (see time chart). Whenever a pressure switch trip is detected
in one of these APF intervals, the control executes a 10-second “APF purge” to clear the pump of soil.
These APF purges occur in parallel with the cycle and do not interrupt or affect the timing of other
functions (like washing) that are called for in the interval. Each APF purge consists of 5 seconds of Fill
and Drain followed immediately by 5 seconds of Fill by itself. If an APF purge is executed during a
heated wash interval, the heater must be turned off during the first 5-second Fill and Drain portion of
each purge, but cycle timing is not affected and the heater turns back on mid-way through the purge.