Wolf Trip is a Ranger Utility power available at the 2nd level.
Wolf Trip
Ranger Utility 2
Once an enemy is on the ground, your wolf companion excels at keeping it there.
Encounter ✦ beast, martial
Personal
Prerequisite: You must have a wolf beast companion
Requirement: Your beast companion must be adjacent to the triggering enemy and able to make opportunity attacks.
Effect: The triggering enemy is knocked prone and cannot stand up until it begins a move action not adjacent to your beast companion.