That's vague on purpose, what works best in your group?
Combat is a fight. I'd say, when both sides have their weapons out and are engaging, even before the first bullet flies... If a character with one of those moves is there, they roll that roll, if they would engage no matter what the result of that roll would be. When you enter combat.
That said, this is something I worked out with my group. Something we settled on before the moves were introduced. But yeah, if the Indomitable enters combat, they can ruin something really fast. It probably wouldn't replace a go aggro, since that's happening before combat under threat of combat. But it would surpass a seize by force roll. I've heard of some MCs here saying that spending a hold isn't automatic, it still takes time. So just cause the guy drops 3 hold during some really fast action, doesn't necessarily mean that everyone else didn't get a chance to get into that meanwhile. These are things I would suggest you chat with that player about, make sure you both feel happy with the move.