We get it - it doesn't make sense. How can you wear this disc and have sex at the same time?!
Here's some information that might help...
The vagina expands and elongates during arousal and intercourse. This allows FLEX Disc to be worn during sexual activities without it getting in the way or coming out.
That is all to say that FLEX Disc should be comfortable for both you and your partner during sex- from oral to penetrative sex. There is a chance that one of you may feel the rim but there should not be any extreme discomfort. How much you or your partner can feel FLEX disc can depend on your position and you and your partner's unique anatomies.