The vagina has very little nerve endings. Orgasm comes from the clitoris so you're actually stroking the clit from the vagina to get her off.
Bit like guys getting off from prostate massage. You're stroking the prostate from the anal passage
https://www.google.com/url?sa=t&sour...LYH87P81kWbl-9
You don't have to pound her, just stroke the clit from the inside