Hitchhiking in the context of a porn video refers to a sexual activity or fetish where one person is "picked up" by another as if they were hitchhiking, essentially indicating that one individual is seeking assistance or favor from another. In this scenario, the term could be used metaphorically for any type of sexual encounter between two or more people where one party takes control over the other.
In the porn video context, it implies a scene in which someone might stop their car to "help" a hitchhiker (or someone who appears to need help), often leading to a sexual encounter or interaction. The power dynamic involved, with one person taking charge and the other submitting, can be a significant part of the appeal for some viewers.