That's great herschel, except it doesn't explain the bajillion other abilities that can slide larger creatures.
Let me throw a couple of examples:
Footwork Lure:
"Modo the dwarf fighter swings his axe in a large arc, hitting a glancing blow on the wyvern's beak, but apparently leaving his flank exposed.
The wyvern, seeing a good opportunity, claps her leathery wings and lunges forward the bearded bastard.
Suddenly, the dwarf smiles and uses the momentum from his axe blow to jump sideways, moving out of reach of the wyvern's claws. The wyvern, however, is not so nimble and moves forward a few feet before braking with its wings..."
Bait and Switch:
"Conina the rogue opens her arms, in a "come hither" gesture that infuriated the hill giant, which charges forward, trying to crush that pesky human.
Conina however, rolls forward between the giant's legs, slashing the inside of the thigh. The giant stumbles forward while Conina rolls through the other side in a gymnast's flip..."
You can also see an example of Deadly Positioning in the art in page 122