On Mon, 12 Jul 2010 11:05:38 -0700 (PDT), wrote:

When you smack the crankshaft, the flywheel
will want to tend to stay in place, while the rest of the lawn mower
will go in the direction of your hit.

So that makes three (somewhat similar) explanations so far:
1. Vibrations allow the friction to lesson
2. Move the crankshaft down 1/8th of an inch moves the flywheel up
3. Try to move the crankshaft down; it can't; so the flywheel moves up