| He means "tetris shape". The standard crescent key (aka "woodruff key") looks like a half-circle. the advanced-timing "rocket key" looks basically the same, but with a ridge right down the middle, and the top offset a little from the bottom. Follow that link to Dave's Motors - that's it.
Generally, on several other forums, I've heard both good and bad things about adding a rocket key to your Cag. The mass majority seem to lean towards NOT adding it, as it makes the motor run too hot. | 
