Dont have examples from JAV really, beside some ARM or AARM handjob scenes where most scenes are handjob only, but some girls suck on it. From western side strokies is a good example. Most scenes are handjob only but the wilder the girl is the more chance they will go the extra mile. Eden sin and ziggy star or vanessa vega scenes for example where the girls are really enthusiastic and make it a BJ scene when its supposed to be hj only. Obviously theres a chance its scripted but it's still hot because most are HJ only. At the very least it gives the illusion that the girl was so turned on she went the extra mile.
The hj to bj is just an example, the allure is the crossing the barrier and going off script because the girl is so turned on. Could be a no sex scene with only a thighjob but pops it in for a sec.
I only occasionally pop in for JAV when im in the mood but searching for stuff is insanely difficult, especially niche stuff like this. I really miss r18, was the GOAT site to search jav stuff.