You know the typical reason why one would jump a 12V battery, right? It's like what happened to the Dread Pirate Roberts after being in the Pit of Despair: it's
mostly dead. So you find a friendly person with a car, jump your battery and while you got your car running and the alternator is charging the now partially dead 12V battery, you pull off one of the battery terminals and see if your car continues to run or stumbles to a halt.
Yes, in the old days this test might have worked. However, with today's systems you may have blown the regulator inside the alternator. Oh yeah, since the battery was
mostly dead it's now in a charging state so that means current is flowing into the battery. When you interrupt this connection by pulling off one of the leads, negative is what was usually recommended I believe, there's probably going to be a
spark. What's usually nearby a charging lead-acid battery? Oh, this image comes to mind:
View attachment 592331
So, no, I would not recommend pulling off either one of the battery leads when the engine is running.