It 1 tick refresh under 50% MP. Only counts MP+ gear in the following slots though: Main,Sub,Ammo,Head,Neck,Body,Hands, Waist(Itself),Legs and Feet. MP% Gear did not trigger it.
Source myself spending 40 minutes swapping gear around triggering.
To add on to Milkmanskid's post, any -mp gear won't affect this item's refresh either if it's not in those slots. My refresh stopped at 575 out of 1126mp, or 51%, with an Aenotherus mantle on. Without, it'll stop at 575 out of 1156mp, or 49.7%.