ok, they do exactly what it sounds like, they grind away a few thousandths of an inch of material off the deck of your head each time it is surfaced. So yes, it will eventually affect your compression ratio. you may want to look into getting a different piston with an even lower compression ratio if you must have this done again.
__________________ Sometimes I think that government fits that old-fashioned definition of a baby: An alimentary canal with an appetite at one end and no sense of responsibility at the other.
- Ronald Reagan
Stangless...for now | 
Resurfacing Trick Flow Heads - How many times? 
