When this feat is activated, a melee special attack is made. If it hits, the victim is dazed for 18 seconds normally or 30 seconds from a successful sneak attack. (Whether or not your character can normally perform sneak attacks) or until they are damaged. Unlike Trip and Stunning Blow, no save is made against this effect, as of Module 4.
Creatures who cannot be sneak attacked will not be affected.
Sap works very well in epics.