Ben Wa balls are definitely used to strengthen the kegal muscles and can be used as a sexual stimulant as well. There are several different sizes and you should start with one the size of a larger marble. Just one though! The muscles in your uterus will contract to hold the ball in place. Do this while you're at home - vacuuming, in the shower, cleaning, etc. Once you don't feel you have any issues holding the ball in place, try two of them. Once you've mastered those, try a larger size and so on. Don't wear them out in public and not for any type of vigorous exercise because they may fall out. To get them out just spread your legs slightly and place your hand down there to catch them when they fall. Sometimes you may have to insert a finger but not often. They will not get stuck and they will not get lost.
They can be stimulating for a man during sex as well but again, just start with one. Some men find it enjoyable and some do not.