I found this. Basically what Ed said.
the Vortex's prongs resonate sort of like a tuning fork and create turbulence, disrupting the gases that cause flash signature. The Vortex is manufactured by Smith
Seems to me, this is the same reason why the original 3 prong M16 flashhider worked better than the current A2 birdcage, which is designed to be both a compensator and flashhider.
From my understanding, a muzzle brake/compensator only redirects the gas to decrease muzzle jump and lessen felt recoil. A flashider either disrupts the gas flow, (as in the above example) or allows it to expand (hence, lowering the pressure) before being allowed to exit fully. Its all in the design.
Thats my best guess.