An observer and/or participant in an intercourse session --- often the "second"
guy/
girl in a threesome --- who is tasked with promptly tucking the dude's randomly-swaying woodie back into the chick's love-tunnel whenever it accidentally pops out from his having unintentionally withdrawn it a bit too far prior to his next thrust; this relieves the lovers from irritating pauses in their steamy copulating to semi-blindly fumble back there themselves and
re-insert the errant
schlong each time.
Finding a willing re-insertion assistant is usually quite easy --- often, you simply need to approach a
random passerby and offer him/her a "standard" reward for his/her help: agree to allow the person "a turn of his/her own" with the opposite-gender intercourse-partner afterwards (or at least a post-session hand-job/
blowjob from the gal if it's a male assistant), let the person play with the balls and
butt-cheeks of the copulating duo for a few moments after each occasion during the "
hot 'n' heavy" when his services are required, and/or
maybe allow him to give one or both partners a full-body massage --- extra points if you both also give him a nice soothing rub-down in return, of course --- after the session is over.