I was underhunting with a friend when I discovered that rebuke doesn't knock down any target using a shield, regardless of skill. Just ran a quick test with a PC to try and confirm.

First cast from my Paladin's PoV (50 mana, straight cast and fully targeted):

You rebuke xxx with your pavise shield.

You feel a twinge in your soul, but since you are in a challenge you have been pardoned.
Waves of dull golden light emit from the shield which crash over xxx.
xxx manages to deflect some of the first wave with his gargoyle-hide shield.
The first wave inflicts a glancing (1/23) strike to xxx right hand.
The second wave passes harmlessly over xxx.
xxx manages to deflect some of the third wave with his gargoyle-hide shield.
The third wave inflicts a glancing (1/23) strike to xxx chest.
xxx manages to deflect some of the fourth wave with his gargoyle-hide shield.
The fourth wave inflicts a glancing (1/23) strike to xxx abdomen.
xxx manages to deflect some of the final wave with his gargoyle-hide shield.
The final wave inflicts a glancing (1/23) strike to xxx back.

Roundtime: 1 sec.

Target stance:

Viewing current combat stance...
You are currently using 100% of your evasion skill.
You are currently using 54% of your shield block skill.
You are currently using 30% of your weapon parry skill.
You are attacking with 100% of your offensive skill.

The cast took about 26% of the targets vit. We did a couple of tests in that stance, which yielded the same result. The target never fell to his knees or to the ground so we tried a full 0 stance on the target.

50 mana fully targeted straight cast from my paladin's PoV:

You rebuke xxx with your pavise shield.
Waves of shimmering silver light shine from the shield which crash over xxx.
The first wave inflicts a glancing (1/23) strike to xxx head.
The staggering force drives him to his knees!
The final wave inflicts a glancing (1/23) strike to xxx left arm.
The overwhelming force crushes him to ground!

Roundtime: 1 sec.

This time the knock down worked but, oddly, the vit hit was 10% with the target stanced at 0 so we did another cast with him stanced at 0%...

You rebuke xxx with your pavise shield.
Waves of shimmering silver light shine from the shield which crash over xxx.
The first wave inflicts a good (4/23) hit to xxx head.
The staggering force drives him to his knees!
The second wave inflicts a good (4/23) hit to xxx head.
The overwhelming force crushes him to ground!
The third wave inflicts a glancing (1/23) strike to xxx right leg.
The staggering force drives him to his knees!
The fourth wave inflicts a glancing (1/23) strike to xxx abdomen.
The overwhelming force crushes him to ground!
The final wave inflicts a glancing (1/23) strike to xxx left eye.
The staggering force drives him to his knees!

Roundtime: 1 sec.

31% vit hit that time so last cast was just a bad roll but... The target was left kneeling instead of on the ground because he was hit with an odd number of knockdown strikes greater than 1. Next, we tried making the target's shield stance 1%, while leaving evasion and parry at 0%.

My paladin PoV (50 mana, fully targeted, yada yada):

You rebuke xxx with your pavise shield.
Waves of dull golden light emit from the shield which crash over xxx.
xxx manages to deflect some of the first wave with his gargoyle-hide shield.
The first wave inflicts a glancing (1/23) strike to xxx neck.
xxx manages to deflect some of the second wave with his gargoyle-hide shield.
The second wave inflicts a hard (6/23) hit to xxx chest.
xxx manages to deflect some of the third wave with his gargoyle-hide shield.
The third wave inflicts a hard (6/23) hit to xxx chest.
xxx manages to deflect some of the final wave with his gargoyle-hide shield.
The final wave inflicts a glancing (1/23) strike to xxx left arm.

Roundtime: 1 sec.

No knock down at all with 1% shield stance and 0% everything else.

Relevant skills...

Caster:
Targeted Magic: 560 67.71% scrutinizing (17/34)

Target:
Shield Usage: 252 29.08% clear (0/34)
Evasion: 353 73.56% clear (0/34)
Parry Ability: 248 79.87% clear (0/34)

The caster has more in every stat, but I can post specifics if necessary. I don't count on rebuke for the damage, especially since it's buggy right now, but I do really enjoy the knockdown aspect of it.