They're valves which are domed out or flat instead of dished in... this raises the compression (minimally) the low comp. valves are just standard designed valves (like stock ie dished)...
The problem with this is that
A. it can TOTALLY screw up the a/f swirl design of your comb. chamber
B. it raises the comp. like .3-.5...oooo that ought to help out a lot
the nitride coating and the fact that they're forged valves makes them better imo not the comp. raising.
Try searching google before you ask Q's... I found that out in like 3 seconds after looking at what you were looking at on nopi.
P.S. here's what I read...
http://www.inlinefour.com/skunracval.html