[Carnut427]

Simply put, in a performance cam, the intake valve is still open while the piston is on the compression stroke. While the intake valve is open, the engine cannot compress the fuel/air mix. The wilder the cam, the longer the intake valve is held open, lowering the "dynamic" compression ratio and thereby allowing more static compression, which in turn raises the dynamic compression. Iron heads get by on around 8:1 dynamic, and aluminum heads up to somewhere around 8.4:1 using premium pump gas. More quench (or squish) you have helps, as does rounding any sharp corners on your heads (combustion chamber to deck area) and pistons (valve reliefs, dish to flat areas and/or domes).
Hope this helps,

Dan