I've been playing with a guy who believe's that it's possible to use Bull Rush and Hydraulic Push to move people vertically, either launching them into the air or knocking them to the ground.
Bull Rush Excerpt: A bull rush attempts to push an opponent straight back without doing any harm.
His argument is that up (or down) and away is still "straight back" by definition. Bull rush is pretty easy to argue that this interpretation is wrong, but Hydraulic push is the real aggressor as the effect can appear from any direction.
I know the rules are only assuming horizontal movement in this situations, but I'd like to back it up with text and I can't seem to find anything specifically saying that forced movement is relegated to the horizontal plane. I assume the only exception to this is when you are physically above/below your target, such in the case of flying or borrowing.
We're a pretty RAW heavy group, hence I'm looking for a very RAW focused answer.
Thanks in advanced.