The scientific babble would be about Newton's law every action has equal and opposite reaction. If you make the weapon heavier, the rearward velocity must be reduced to keep the formula in balance. M1V1 = M2V2 . Mass1 ( The projectile) times Velocity1 (Initial projectile velocity) equals Mass2 (Weight of the weapon,the caseing, etc) times Velocity2 (rearweard velocity of the weapon, caseing etc). Less rearward velocity equates to less felt recoil.
The fluid fills less that the availave volume. The fluid's initial accelleration and density absorbs some energy further reducing 'felt recoil.
For any given cartridge, a heavier weapon has less felt recoil that a lighter weapon. Which is why the M1 garand is such a fine shooter - its heavy! Much heaver than my H&R HandiRifle in 30-06.
I have one for my Tactical Nova I use it during practice exercises - It does add weight to the weapon making the Nova heavy. I remove it after the exercise. When many rounds are going ot be expended during a session, the unit helps my aged shoulder with rotator cuff problems...
Hopes this helps -
Rick