They are in the mood for some rough play, so Joe is domming Nicky or just topping while they have rough sex, when Joe gets overwhelmed or uncomfortable or wants to stop for whatever other reason, and uses their safeword.

Bonus but not necessary to satisfy my kink: Joe ignoring his own needs and emotional fallout because he's so worried about leaving Nicky hanging mid-scene and apologetic that he used the safeword when it's "supposed to" be for Nicky's use (it's not, any participant can revoke consent for any sexual activity anytime! I know this, I'm just saying for the sake of my twisted fantasy that's the mindspace Joe's in ;)