Gravity.
No wait, that can't be right. It is the set screws - you adjust them to reset zero just like with other scopes.
But I will admit not being all that familiar with the M-series. They don't pay me enough to actually own one.
Our tech support team might have a more in depth answer, and they're a bit slow this time of year. Give them a ring. 888-550-6255
Geoff at Burris and Steiner