The video shows that the cop had his knee on Floyd's neck for most of time in the restraint position, then he move it back a little to Floyd's top shoulder.
He probably blacked out due to both having air blockage and blood flow restriction with that amount of pressure on his neck area as well his chest by the other officers.
When this happens its the police's duty to take assertive action to revive the person who was placed in restraint.
All the cops at this arrest event screwed up and they will most likely be punished with Derek Chauvin receiving the highest level of punishment.