I'm guessing that mercury recoil reducers work using the fluid properties of mercury.
You get two benefits:
1) increased mass, which will reduce the felt recoil just like any other similar mass
2) delayed recoil, because the liquid mercury will stay in one position momentarily while the gun recoils around it, then the mercury will recoil. This will spread out the total recoil over a longer period of time, reducing any momentary felt recoil
*though I make no claims regarding my intelligence vis a vis yours.