Original: The plan was executed when the match referee, Earl Hebner, under orders from McMahon, called for the bell to ring and ended the match as Michaels held Hart in the Sharpshooter submission hold (Hart's signature finishing move), even though Hart had not submitted.
Simplified: The screwjob happened when Hart's opponent, Shawn Michaels, applied Hart's own finishing move, the Sharp Shooter and  then WWF Referee Earl Hebner called for the bell under Vince McMahon's order even though Hart had not submitted (or tapped-out).