You can't gain momentum from hitting with swift riposte.
You can make a basic attack against your attacker. If your natural attack roll equals or beats your attacker's roll, resolve your basic attack against that enemy first. If your attack roll is lower, your attack has no effect, regardless of whether it hits or misses.
If your swift riposte attack is a critical hit, the enemy's attack misses.
You gain a +2 attack bonus with swift riposte attacks.