I have grooved a couple of dozen engines, mostly gen I, II & III SBC for high performance use. The groove alters squish flow to provide a high velocity stream of turbulent air fuel mixture into the developing flame kernel. The effect of the turbulence is to promote mixing of air and fuel into a more homogenous mixture that burns more quickly and efficiently. Here's a set from a 383 LT1 street engine I'm finishing up.
The people at
www.mpgresearch.com and
http://forums.turbobricks.com/ are using the idea to reduce fuel consumption with 4 bangers.
My favorite project was Paul Pitre's Super Gas car; powered by a grooved 355 CI SBC.
9.90 ET at 140 MPH on the throttle stop, idles at 600RPM cold.
Low 9's off the stop

Cheers,
AB