It only negates the knock-back affect on abilities that knock you back as a result of doing damage, or so it seems. So if you negate the damage portion of the attack, the kb doesn't kick in. So it also works on the Ifrit move that pushes you back if you're in range, for example. With AK it's the inverse; you get repelled first and take damage after. I haven't tried it on Garuda's whirlwinds.