Sacrifice - how to use properly?

[Petwin]

You can use sacrifice on a stack of droids. A (different or phantomed) stack of repair droids can repair (resurect) the sacrificed droids. The regeneration of Trolls at night gives the same result, but won't bring back a killed Troll. You can use the spell Peacefullness (order magic) to increase the hp of the stack you are going to sacrifice.

Basically: with repair droids or Trolls you can grow your army for free.