Just a side note for less experienced readers.
Recoil doesn't change, it's a measure of force. The powder charge pushes against the breach of the gun in one direction and against the bullet 180 degrees in the other direction. Recoil is effectively just Force = mass (bullet) * acceleration (gas expansion). Other factors come into play such as mass of the gun, which is why different guns feel different when firing.
But you're correct in the 'perceived reduction in muzzle flip'.
That's what the marketing materials call it, because there is no known way to measure it subjectively against all known firearm makes/models and all known loads of ammunition. As well, the muzzle flip is unique to each person: grip, strength, technique, etc, all have an impact on perceived flip.
That said, pure physics says a heavier object further away (muzzle) on a perpendicular plane along the axis of the fulcrum (wrist) will require more force to move, implying a heavy RSA made of materials like tungsten or steel will negate some of the forces that drive the muzzle flip.
Additionally Glocks are preferred over other brands because the barrel is lower and closer to the axis of the forearm, reducing the forces need to offset the muzzle flip. Because its not perfectly aligned with the axis, there will be some vertical forces acting on the muzzle (lever) where the wrist (or poor grip) would be the fulcrum.
Most revolvers have more muzzle flip that most automatics because of the height of the barrel, using the center of the wrist as a measuring point.
An 'ideal' (theoretical) firearm would have the breach and muzzle perfectly aligned with the center of the fist, and if the wrist is perfectly aligned with the axis of the forearm there would be, effectively, zero muzzle lift, and the recoil would be offset by the added mass of the 'perfectly aligned' hand, wrist and forearm of the shooter.
