Pinch is the force you feel when you turn the crank around without glowplug mounted. These engines have no pistonring and the solution is a conical shaped sleeve with a slight oversized piston you have to run in zo they will shape to each other perfectly on the right temperature.
In cold conditions with most enginen some pinch is a good sign because on temperature the pinch is gone due the unequal expansion of the piston and sleeve.

Pinching is a methode to make the upper part of the sleeve smaller using a press arround the sleeve. This is only needed when the set is loosing compression.