Any ships you have at a world, whether they are D-Ships, or ships on fleets, that were not already moving or firing, will normally fire automatically at any enemy's fleets that move through the world without stopping (that is, fleets that move in multiple hops at once, enroute to a different world).

When ships fire in ambush mode, they do double damage: each D-Ship or fleet ship that ambush fires can destroy one unloaded ship.

As if that weren't enough, ambushing ships can fire at multiple different targets in the same turn--so if an enemy attempts to bring more than one fleet through your world without stopping, your D-Ships and fleets at that world will ambush fire at each of them, doing double damage to each enemy fleet.

There are some very important things to understand about ambush. First, it only applies when an enemy attempts to pass completely through a world enroute to another world, using multiple hops. That is, only the ships at the fleet's intermediate stops, if any, can ambush. The ships at a fleet's final destination will not ambush fire. (You do not have to be the owner of the world in order for your fleets to ambush fire at that world.)

Second, only those ships that did not not receive an explicit move or fire order will ambush fire. If you ordered a fleet to move, or if you ordered a fleet to fire at some target , then that fleet will not ambush that turn. Similarly, if you ordered your D-Ships to fire, then they will not ambush.

It is possible to turn off the auto-ambush feature at any particular world or worlds, or at all worlds you own, for a given turn. This is only very rarely done. You might do this, for instance, to extend safe-passage to an opponent through a particular set of worlds.

The ambush rule helps you to defend your inner circle of worlds with D-Ships, forcing your enemies to approach slowly, one turn at a time, or risk being decimated by your ambush fire.