Munin has the right of it big time! Acting Under Fire is triggered by taking risks in the game's fiction, but it's actually a generally safer move than, say, Seizing By Force, especially when you have +3 Cool. Playing a Battlebabe entitles you to take audacious risks and, more often than not, come out the other side unscathed.

I would add that filling out details of the environment and explicitly providing the PCs with opportunities can be a good way to let the Battlebabe shine. I think that, for players who are accustomed to the combat of other game systems especially, they may not see themselves as having that many options. They're stuck in the mode of "Attack, attack, attack" one enemy at a time. That's not where the Battlebabe shines. It's when they come at people sideways and find ways to attack opponents that cannot readily fire back that they shine. If they're in a situation where they would have to Seize By Force, consider how they could Act Under Fire to turn that "Seize By Force" into a "Go Aggro"--and blur the line between "combat" and "negotiation" while they're at it.

But I get the feeling you already get that; the problem is that your players might not. That's why I say explicitly provide opportunities. While it might seem obvious to you based on your description of the scene that the hanging chain would let the Battlebabe swing behind the gang leader, it might not occur to your player. So, ask a leading question like "There's a chain hanging there, and if you swung on it you could get behind the gang boss--if you can avoid getting blasted into oblivion on the way down. Do you wanna?" These sorts of leading questions can help teach the game, communicating to the players the types of moves that are available to them. Then hopefully next time they'll start springing forth with ideas like that on their own.

The other side of the equation, though, is a little harder to address: motivation. The game sings when the player characters are highly motivated and really go for their drives. Unfortunately, the Battlebabe can sometimes appeal to players who just want to hang back, stay safe, and not care about anything. Getting a player like that invested and engaged enough to take action can be a challenge but not an insurmountable one when you meet them halfway.