While having a shield equipped in the shield slot on the left arm, select shield bash from the combat stance bar (or bind SHIELD_BASH to some key). When you are successful, you get the message that your opponent "is sprawling on the ground".
The quality of the shield seems to make no difference to the chance of knocking down your target. Also shield bashing skill does not affect the duration between bashes.
A successful shield bash stuns the opponent. The length of this condition varies, but there is no known maximum stun duration.
Depending on your shieldbashing skill and the defence of your opponent, you may also do a slight bit of damage. Factors in the damage include you and your opponent's body strength, as well as what armour, if any, the person being shield-bashed is wearing.