first off, I agree with everyone else. It can be a dangerous thing, a rolling horse, plus, its kinda hard on a saddle.
Let me add this...go watch your horses in their pens. Every horse has their own way of going down to roll. Learn their pattern and then you will know by their behavior when they are getting ready to do it.
Some friends were at the indoor arena a couple of weeks ago, and I had our newest gelding with me. The guy we are buying him from was there, so I had him get on "Splash" first, so I could observe his riding action. The dirt in the arena was freshly turned, so it was very soft and fluffy. Right away Splash pawed a couple of times, put his head down, and then before anybody could say anything, he was down on the ground rolling. David got clear of the saddle, and started poking Splash, and he got back up.
So then I got on him a few minutes later, and immediately he started doing the same with me. I realized right away what he was doing so I kicked him, to disengage his hindquarters, and got him moving. He didn't try it again.
You would have thought after the first time, David would have recognized the signs what was coming, but as soon as he got on his own horse, it did the same thing!!!
So after all this, i've started watching the two geldings in their pen, so that I know ahead of time, what their signals are. Then I can stop it before it happens.