Did you hit anything solid with the blade? Sounds like you have a sheared flywheel key - this is an intentional fail point, and is designed to prevent the engine being re-started until you have checked for a bent blade/crank. The cord is being yanked out of your hand because the effect of having the crank stop dead and the flywheel continuing to rotate (thereby shearing the key) is to advance the ignition, causing kick-back. Don't continue to try without resolving it - the starter assembly will eventually fall apart - if your arm doesn't first!
Phil Saunders, November 2007
