The correct or another term is quench . Opening up the chambers on the outer edges of valves is a good way to lower comp as it's bassically relieving valve which increases flow... Quench is not such a big deal with forced induction as it is with hi po n/a engine... But the idea is to get one big flame front and not many, as this causes detonation... Note the 351m / 400m motor in the States is notorious for detonation yet it has approx 8.5 compression and Clevo style open chamber heads.. The main reason for bad detonation?? The piston is designed to be way down bore .. The fix to this was to fit pistons with pin height lower essentually bringing piston closer to deck. Even though it raised compression it prevented detonation...

exactly the reason i don't like thicker head gaskets and head shims

nothing like a GOOD tune.
but you shouldn't just realy on the tunner. when the engine is built you should make every effort you can to decrease the likelyhood of detonation.

Yes I think I may have got alttle carried away with 27cc dish with 61 cc chambers on my 330W strocker though...BUT it gives you every oppertunity to make BIG power...7.8 compression!!!