Pelvic floor muscles. The “grip” strength can give her much more power to climax. From your description, there is some chance she is unfamiliar with kegels and such. If so, definitely worth exploring.
Also: men can strengthen pelvic floor too, and many women enjoy the internal throb/pulse feeling, so it might be something to try together.
Agreed just FYI you probably mean “encourage” rather than “convince.” If they don’t want to explore their body for whatever reason, OP shouldn’t attempt to convince them.
(That said, if they are uncomfortable with their own body/sexuality, it might be something to bring up with a mental health professional, since there’s usually a lot to unpack there.)