In the 0.11 updated for Minecraft Pocket Edition, Mojang introduced "StrangePacket", which allows server plugins to transfer players between servers easily.

The plugin FastTransfer (By one of the MCPE devs) took advantage of this so you could do /transfer <player> <ip> <port> and be instantly transfered to that server.

However, this was removed in later updates and still has not been added back. This is essential for large server networks, as you can change to another server by walking into a portal or tapping a sign.