the obligation is to stop applying force when the attack is over or the threat is perceived to have stopped.
Much easier said than done in some real life scenarios.
It is not a sport with good sportsmanship, but in court it is judged that way.
If you put someone down twice your size who attacked you, they seemed quite competent, and they are in a position to get back up and probably will do so...
Do you have an obligation to wait until you once again are possibly at a disadvantage, they are in a position to do damage and you may not get so lucky again, or do you go further and do something you know will keep them down?
Do you have an obligation to let any attacker get back into a nuetral position before continuing with the momentum and advantage you have and may not get again?
I know the law. But if you succeed a couple times, but they get right back up, they may eventualy succeed and not be so sportsmanlike when they have you in a compromised position.
Unlike the movies many serious fights have moments where one or the other gains an upperhand, with that upper hand they can do something that puts them in control of the outcome.
If when you are in control and you then back off, the guy recovers and then resumes attacking and gets in that position themself they may not be so nice. It can cost you your life.
Now in a court of law finishing someone you downed will get you sent to prison. In real life not doing so can in some situations get you sent to the morgue or with permanent life altering injuries.
You don't get to ask to do it all over later if they incapacitate you afterwards and proceed to hit you with something, stomp on you while you're down, or otherwise continue thier own attack resulting in your disability, disfigurement, or death.
With a firearm it is different because the result can be decisive. If you shoot someone, you can stop, and if they pose a danger again you can shoot them immediately again, without being in much additional danger.
In an unarmed fight something that puts someone down is not always decisive, and advantages not taken advantage of may be opportunities lost that you regret when later at the disadvantage yourself.
You don't just get to pull a trigger again if they resume the attack after you stopped the attack, you get to resume trading attacks. Resulting in a much less certain outcome.
That is one reason a firearm is so much better, public opinion understands thier use. You can defend yourself, stop, and if put in danger again defend yourself with the pull of a trigger.
You can do only what is necessary to stop the attack if you do have to fire and then evaluate the situation.
You have clear lines when to attack and when to stop attacking without posing significantly more risk to you.
You do not have that luxury in an unarmed or knife fight with a determined attacker, or multiple attackers that want to do serious damage. Yet you will be judged the same in a court of law, expected to put yourself in danger to let them resume the attack if you manage to stop the attack briefly.