The time-sensitive pattern created by that pinch-circle motion, along with the motion involved in rotating the dial, are complex enough that Memon says a study of about 100 users has shown it can identify users with about 2% false positives and 2% false negatives.
The umpire will signal the start of the fielding side's choice of powerplay by waving his arm in a circular motion - and indicate a powerplay chosen by the batting side by waving his arms in a circle and then clapping his hands above his head.