Yes, they knew the new mechanics. They did not know the precise number, but in the same way one might give narrative hints about how wounded a creature was, I gave them hints regarding how close to filling that hit box they were. After the hit box was filled, I told them how much damage they had done. So in the first 3 fights, where each boss had 3 equal hit boxes, they were able to sort of hone in by hit box 3.Did the players know the new mechanics? Did they know they had to do 150 HP /rnd in order to make progress toward defeating it?