The chick(s) who sucks the porn stars' dicks in between scenes to keep them hard (rarely the same person who sucks his dick on-scene).
Many porn flicks displays scenes of animalistic sex for up to 45-60 minutes. Do you really think the girl (or guy) can last that long? They actually take a lot of breaks, with several girls coming from behind scene to keep the guy aroused. These are the buffers.