overheating and just plain use. what happens is your sleeve expands when it heats up to running/racing temp. now when the engine gets cold, the sleeve contracts. this so far is the normal process. now, when you overheat the sleeve, the sleeve overexpands and when the sleeve cools down, it will not contract all the way. thus, loosing compression. that is where pinching comes into play. since the sleeve has not contracted fully, it has a slightly larger inside diameter now. that is why when you turn the flywheel over, it is easier to turn. the piston is relatively the same exact size but its going up through a sleeve that is larger and making little to no resistance at the top. now you can pinch the outside of the top portion of the sleeve to give the compression back. a lot of guys do this.
hope this helps.