Originally Posted by rmdhawaii

That tightness helps maintain the intended compression, so that you get the maximum downforce after combustion to push the piston down. As you lose the tightness due to engine wear, the compression decreases on the upstroke and that is how you lose power. Compression gauge is a good tool to have if you suspect you're losing too much compression.

The longer you maintain that tighness (aka pinch), the longer your engine "lasts" so to speak.

This is why some people believe that WOT is the best way to break-in an engine if WOT is an option. Not all are designed for WOT break-in. WOT breaks in the metal way different than if you just let it sit there and idle. The theory is, that WOT maintains the pinch longer.

I broke in my Sirio STI and OS TZ (photos) using WOT with excellent results, although the STI is slowly losing it's pinch. I also have the Orion CRF and it is not a WOT eligible engine. That engine is super tight and they don't recommend that you take it out the piston or clean the surfaces either - ha ha.

Hope this helps explain things a bit.