On Barbarian, when the Ring of the Ravenous unique is equipped, the Concussion passive does not work for brawling skills like Leap and Charge (with two handed mace assigned).
For example, my barbarian has 100% calculated chance to stun with Leap using the Concussion passive, and in testing it stuns every time when Ring of Ravenous is unequipped.
When Ring of Ravenous is equipped, Leap will never stun from the Concussion passive. This is due to the fact that Ring of Ravenous causes the weapon to quickly swap from the mace weapon to the slashing weapon in the animation.