Hardcore insert

In the terminology of movie-making, a hardcore insert (also known as piston shot) is an insert, depicting penetration in a film. The technique was much in use in the 1970s, when Pornographic films were feature films that required women with acting abilities, at least for the lead roles. Many actresses found it unacceptable to perform actual sex (or at least anal sex), so the inserts were shot with body doubles. There was hardly a need for inserts in scenes of fellatio and cunnilingus performed by actresses, because the depiction of oral-genital contact was then considered "semi-softcore". Some people believe that one Swedish actress, Marie Forså, used to have actual sex, but then the penetration scenes would be removed from the cut and replaced with inserts.

More information is available at [ Wikipedia:Hardcore_insert ]