The way the ability works is that the 50% health cost happens at the end of the abilities duration, not based on when the ability activates but when it ends. That means in simple terms, under the best case scenario, when the ability ends you will go from 100% to 50% health.
It would of been better to put a healing percentage decrease on it or no healing in general till it ends. Putting the health drop after the ability ends makes it useless in pve now and in pvp it just means marauders are going to save camo for after they pop UR and get healed during camo or simply just run away.
This really wasn't a well thought out change and was a mechanic I was hoping BW would stay away from and maybe change with endure pain for juggs which also has a HP drop off after the ability ends which, again, I think is a terrible design that is now being implemented into another ability.