the only reason i can think of that the seal would be broken when the cup is full is if the entire area between the cup and the cervix filled with blood, and then you bled more. in this case i could see the pressure breaking the seal, but this seems like a very rare scenario. even in this case it doesn't make a whole lot of sense, it seems like the cup would just get pushed down. this is of course if your cup actually sits as low as in the diagrams. if your cervix is really low i could see this happening easily, the filling of the whole area, but i still don't understand why it would leak unless it got pushed down so far that your pubic bone misshaped the rim.
does anyone understand this better than i do? i am just curious because i keep thinking about it and realizing that i just don't get it. hope my question makes sense! i guess i am also paranoid about leaks, and i don't understand why they ever happen if your cup opens up properly.