Well the way resistance works, is that the first knockback will always generally knock you, or at least move you if you have high resistance. It's then SUPPOSED to apply 1 stack of "knockback resistance", that scales very strongly with your usual resistance to knockback. Now, if you get knocked a second time your supposed to be knocked for less, and automatically gain a second knock resistance which reduces it further, the third knock is the last in a chain you are supposed to see any effect, even with low knock resistance this is supposed to be minor.
After that your supposed to have knockback IMMUNITY for ten seconds, before it resets. Alot of mobs though, outright ignore that, viper especially.
At the same time knockback resistance is supposed to be applied from block rating, ie 200 resistance for normal blocking, 250/300/360 for rank 1/2/3 block powers respectively. But that doesn't apply UNLESS you had held it down for the whole time the knockback attack was charging. The whole time its charging you have to be holding block, see a problem? No one can typically do that for a large number of reasons, even though you resist the damage, the knock still knocks you. It used to be if you blocked, didn't matter if it was a hold or knock you were not gonna be affected by a status effect, but they've since nerfed that somewhere along the line. It's one of the changes that really irritates me.