The controller considers the rain sensor tripped when the rain sensor goes from dry to wet.
If I may make suggestion for a firmware update: Should be option to start delay from dry to wet or from wet to dry. I currently have my rain sensor delay set at 3 days. If I have a one day cloud burst, between the delay and my sensor drying out, my water schedule should start back up at just the right time. Recently though, we had a stretch of rain that lasted for 5 days. My 3 day delay was used up during that period and on day six when my sensor dried out ( takes about a day) my system started watering a yard that after 5 days of rain really didn't need it. Just a little more flexibility would be helpful.