This ability makes it so Tarth can expand quickly in the early game. You can make small stacks of units effective and out settle and expand some of the other civs (except Magnar). But in a fight you can take out most of the baddies around your area and build a few cities. Once you reach that point there is no point in trying to get that skill working to greatest advantage.
I generally use it middle game. It is also a great boon, when you make a bad attack decision (or something attacks you unexpectedly) and you start to lose troops. During battle, when you drop below the indicated amount you will get that bonus again. A sweet fight to the death bonus. If you want to get this bonus when fielding large units take a look at Umber.
