Pinching is an attempt to resize the sleeve to re-establish the tight fit between the p&s at the top of the stroke. Pinching is not intended to resurect the dead; but to extend the life of the engine. So, the advise is to do mild pinchs throughout the engine's life. How often? you'd get a different answer from each driver.

Though You will find a great number of drivers who will sware by pinching... I do not subscribe to the practice.