The Backflip is an emote that is only obtainable by purchasing the full Assassin's outfit from Solomon's General Store. Free players can buy this for 1800 RuneCoins and members can buy it for 1620. Currently, you can not obtain this emote any way other than buying the assassin package. If you try to perform it in an enclosed area, it will notify you that you must have 2 spaces open behind you. The emote is actually a backhandspring, not a backflip as the game calls it.