I've just saw https://youtu.be/D_53rB_I3mo on youtube.
So basically:
1) Get a lot of entities in the same place and make sure they can't escape
2) Drop a player near it (make sure nothing blocks the movement of the player as well)
3) Get insta boost
These entities seem to push each other away while they can't (they are blocked in). If an other non-blocked entity gets near, it could get shot away a big distance at insane speeds.
Not confirmed this myself, but at least it should be easily duplicate-able.
can't reproduce in 15w44b unless I'm doing it wrong