Ok heres what it does:
"Adding a flywheel weight will increase the rotating mass, or inertia, of the motor. The result is an engine that is a little slower to rev, has more controllable power delivery and less hard hitting power, a more usable low to mid-range, and is less likely to stall at slow speeds or with the rear brake. What you will feel is better traction when it is slippery or hard pack, a torquier and broader powerband, the top end power on many bikes will be improved with less tendency to lose RPM's when climbing or hitting obstacles. The easier to control power will help you conserve your strength and energy, plus many bikes start easier with the added weight."