Linc is correct. Basically a nitro rc engine creates compression by moving the piston up and down in a tapered sleeve, in lieu of piston rings in a straight cylinder like in an automotive application. Over time the tapered sleeve and piston will wear and eventually not be able to create adequate compression. The term "pinching" refers to a process of taking the worn out sleeve and pressing it into a special tapered die to get a new taper in the sleeve, thus saving purchasing a new piston and sleeve. I find that a "pinched" piston and sleeve generally lasts about 1/2 as long as new, but at a very small cost. Hope this helps