It sounds like you had an episode of supraventricular tachycardia. The medics set the machine to shock at the same time the pulse is going through your heart. This is known as cardioversion rather than defibrillation, though the method and results are very similar.

https://acls.com/articles/shockable-rhythms/

https://en.m.wikipedia.org/wiki/Supraventricular_tachycardia