ben-wa balls are like...well, balls (yeah, honestly!! LOL) that can be made of latex or metal, with, like, another ball inside them, that causes them to move around etc, which are supposed to kind of interact with each other (sorry, i am REALLY crap at explaining anything remotely technical, i am sure there is an official word for the aforementioned 'ball in a ball' thing and resultant interaction, but i know not what it is!).
i have to say though, in my experience, it is more the fact that i am wearing them and feel...um..*thinks*...full, that is a turn on, as opposed to the interaction, although the latter can also be quite nice.
i have heard horror stories about people 'hearing' them...but in all honesty i just can't imagine this happening. i have also heard unconfirmed reports of them slipping out (yikes!), but this has never happened to me in all the many times i have used them.
Hope this helps.
lucy