One thing that I'm puzzling over in Sheldon's description is why the 2x4 only extends about 3-4" past the seat tube. Also, can someone explain why he advocates pushing down on the 2x4 directly over the seat tube and not on the portion extended past the seat tube (pointed towards the head tube)?

Bicycle Frame/Hub Spacing (scroll down to see pics of frame and 2x4)

Can someone explain this bit to me?