Rally Blades appear at the spawn of adventure worlds in the Outpost of Light at the bottom level.
Players can press the "E" key to teleport to a random player, in their current world, that has Rally Blade activated.
Holding "E" allows the player to toggle allowing/disallowing random players to join. Players on your friends list will still be allowed to join your game, and /joinme username will still work, even with this disallowed.
It is useful for challenges to join someone in the right biome.