From what I understand, they are simply a tube filled with mercury, a dense liquid metal. They wiegh around twenty ounces or so, though I am sure they vary a bit. What happens is that as the gun goes off, the bullet is pushed out the barrel at a high speed. Physics dicates that for every action there is an equal and opposite reaction, and the only reason the rifle stock doesn't go through your shoulder at a high velocity is that it wieghs several times more than the bullet. This is why a 220 grain bullet will "kick" harder than a 150 grain bullet, even in the same caliber. They mercury recoil reducers not only add weight to the stock, but puts that weight to good use. The mercury, being a liquid, doesn't move with the stock as the bullet is fired and the stock moves backwards. It will stay at the same place until all the mercury is at the far end of the tube (the end closest to the barrel). At this point the stock has to work at starting the mercury moving from a dead stop, kinda like pushing a car at a dead stop is harder than pushing one already in motion. Also from what I understand they reduce felt recoil quite a bit. And they will not mess up the looks of the stock like a recoil pad will, although a pad is much less expensive.