"Ninjaing" a fleet occurs when another player attempts to attack another with a weaker force. The targeted player then sends their fleet in an escape flight, timing it to return to intercept the enemy ships.

First say someone is attacking you with one battleship and you have enough firepower to know that you will be able to destroy it, and your enemy sends a spy probe to check the coast is clear, so you time the escape flight of your fleet so it arrives just before the enemy's fleet does at your planet. If performed correctly, it should give the illusion to other players that you are offline and unsuspecting an attack, but your fleet will arrive back just in time to "ninja" your enemy's fleet.