Yeah, its a weird little rule... cant say Ive really ever played it though. It needs you to be playing with a serious set of ruins- some that have levels far enough apart (and distinct) so that you can allow them to be seperate levels- remember that if you shoot blasts/templates at a level you can only hit models on that level (and flamers only shoot +/- 1 level)... Ive had some people try some really evil/beardy things because of beasts not going to the top of ruins (like: you can't assault that squad you've been heading straight for because that pile of rubble is the top level of that 1.5" high ruin).