With Jack still strapped in, the back of the guillotine platform comes loose from its structure, and winds up spinning around an axis — with the death machine’s blade repeatedly coming close to decapitating the pirate.
While he’s originally sentenced to get his head chopped off via guillotine (introduced as a new invention), a poorly-executed rescue mission cause these plans to go awry.
In the sequence in question, it looks as though Captain Jack Sparrow may finally pay for his life of crime… but things don’t exactly work out as his captors intend.