It is a sterile (stainless steel I believe)marble that is inserted into the uterous of the mare. The idea is the body will tell the mare it is pregnant and will stop them from coming into heat. We have done a few here at the clinic where I work. I have seen it work great on one mare, did nothing for the second mare and the third mare "expelled" it within days so did not even stay in the uterous of that mare. I guess if it stays in and works, it is a great way to stop heat cycles. Some mare do terrific with a marble, others don't.... Kinda like woman, some can use birth control pills, shots, etc some can't so it would depend on the mare.
I spayed my mare, have spayed all my mares and love the results and the way they act afterwards. To be honest, I didn't want to try some of the shots since some of them are labeled for cows/steers, and just didn't want to have to worry about shots, meds etc and then forget to give them.
Spaying has always worked for me and that is why I like to have that done, and the mares have always remained sweet and loving.