Most of you have played on a server and something that is often on them are launch pads: this pads, usually golden pressure plates, can launch the player in the air, helping him to navigate and explore the hubs.
With entitydata, this could be done very easily in Vanilla Minecraft, but due to the fact that the command doesn't work on players, it's impossible to do things like double jumps or, precisely, launch pads.
But there's a workaround: with a little trick, you can "change" the motion of the player, even if a playerdata command doesn't exist, and so you can create fun and simple launch pads in vanilla Minecraft, with just some command blocks.
How? See the video to understand! :D