Porn stars have very long anal sessions with bigger than average cocks also help the muscles of the anus to eventually tire out (typical shoot can easily last 4 hours+, depending on the number of scenes and how many people involved in the sex). This causes them to completely relax, and "gape" for a little while. A gape is a temporary thing, often times only lasting a maximum of a few minutes depending on the persons body and how long they've been having ass sex (and what size toy/penis is inside them). Some people gape wider, or easier than others. Often times when a girl is interviewed for a shoot she is asked about "how well she gapes"... as this has become very desirable in adult entertainment.