http://www.ws6transam.org/Backcut.jpghttp://www.austincc.edu/wkibbe/headdesign_files/image006.jpgThe latter is an extreme example of a back cut, it's not supposed to be that radical. But demonstrates the area that is cut. (Left valve is back cut, right side valve is not)
Edit: one more:
Shows the back cut (removed material in red) in relation to the port and seat area. You can see how the "ramp" would otherwise create a bottleneck between the valve and the seat. In many/most valve/seat designs I would guess?