You create an area of strange, otherworldly gravity, causing the earth itself to seemingly pull creatures to it. Creatures in the affected area that fail their saving throws fall prone. In addition, these creatures are slowed (as the slow spell) as long as they remain prone, and for 1 round after they stand up. Creatures that succeed on their saves are not knocked down, but are still slowed for 1 round. Creatures that are already prone and fail their saves are slowed for as long as they remain prone and for 1 round after they stand up.
Creatures with multiple legs or exceptional stability (such as a dwarf's stability racial trait) get bonuses to this save as if they were attempting a Strength check to resist a normal trip attempt.