Can it be toggled at will or is their a minute long cooldown? From my understanding what you described sounds like the Soviet self repair in implementation. I may have been mistaken though, if it disabled the building for a full minute before it could be retoggled I can imagine why it should be free.
Especially with the Brits and ML-20 hoard coming the OKW will need the boost.
Cool down, when you activate it for a set amount of time the truck takes 90% reduced damage. While in this state it can do absolutely nothing.