The question isn't so much what would stop momentum as do we want a "block unblockables" shield and if we do what reason could we create that would explain why the best attackers would be the worst?
The answer to that is a gravity shield for attack instead of HP. But, since this is a rare shield series, I am trying to make concepts that haven't been made already, which a concept like gravity shield has.
So your goal is a shield that makes the strongest blows weaker than the weakest blows? (or make the weakest blows stronger than the strongest blows depending on the perspective.)
I would prefer to avoid an unblockable, block unblockable, true unblockable, block true unblockable precedent.
Why not make it so the the shield blocks all damage but deals the user X damage per attack? [X=4|3?]