Allen, I will take a stab at answering your question based on my understanding of Ocroft's method:
The PC rises after a buy is executed. If the trend eventually reverses, AIM will will stop recommending buys, and this is when the PC stops rising. And this is when one executes the (accumulated) buys.
It is a cool AIMish way of detecting a reversal in the trend.
As an example (according to my calculations): If one started an IYE AIM in June this year, and executed monthly based on monthly closing prices, then no buys would be executed until the price closes at or above 44.28.