There are two options. A real tight guard, or a rock no more than half in the top-four.
If the guard is too high (like half way), the other guy has the broom up for the come around a second after the rock comes to stop. Get the guard real tight, then the other guy will have to make a come around that's closer to tee-line, thus 'chocking' off the rings as you see here below. Manitoba threw the yellow guard, Alberta made the come around and then Manitoba freezes to it, chocking off the rings.
Likewise, if you call it in the house, and the rock goes top twelve or anywhere tee-line or behind, then the other guy throws the guard.
What's really fun about this brier is watching that first rock and seeing how the end progresses based on where that first stone ends up. I admit it, I'm a geek, and I should get back to work.