According to the scrubbed fantasy grounds leaks of the subclass, you get a choice of one of these once on each of your turns
- 1d6 bonus piercing damage to one attack
- you experience 5 feet of forced movement
- a creature you hit with an attack must make a STR save or be moved 15 feet horizontally
I was referring specifically to the last one, which due to its wording stacks with thorn whip, although the target has to both be hit and fail a save