Creature is incapacitated until the start of its action phase.
So, it has been knocked down to the ground, and because its incapacitated, it will lose a guard marker, can't use a Defense, and flyers are knocked down and lose flying. As soon as the creature is activated it "stands back up", flip over the Slam marker and they are now Dazed for that round (Daze is printed n the back of each Slam).
So, its a little more powerful than a Daze, but weaker than a Stun (because it doe snot take away their ability to act during their Action Phase). Its a great way to take out guards and render creatures with defenses vulnerable. It has a nice "feel" like a creature being knocked down.