Basically it is tube filled with mercury and placed in the butt stock behind the butt plate. It is supposed to do 2 things. The added weight to the gun should reduce some recoil and the tube is not full of mercury. When the gun is fired the mercury takes up some of the recoil energy as well. At least that is how it should work in theory.
I use one in my Citori and I know the added weight reduces felt recoil. No idea if the mercury itself helps or not.
https://www.midwayusa.com/product/100528...d-7-8-x-4-14-oz
