Pull string snapping back?

On my Toro lawnmower, why does the pull string snap back towards the engine when i attempt to start it? Engine will not start, although it seems close to starting up.
Conrad, June 2005


done
the blade may be loose, or if you hit something, the timing may be out (flywheel key sheared)

peter, August 2005
link Click here to see other fixes for Toro.