Uncanny Dodge II - +1 bonus on Reflex saves vs. traps. Grants immunity to sneak attacks from a flanking attacker
unless the attacker has at least four more combined class levels of rogue, assassin, barbarian, and shadowdancer than the defender.
This is the description on the potm wiki. It's also how the feat works in third edition in general, and nwn in specific too. The in-game tooltip for it, however, has its wording reversed, stating that the defender must have more than 4 combined levels than the attacker.