From: the Netherlands
When attached to a combat unit they are sure to be committed in combat if that particular combat unit is involved. You don't need to roll the dice. The drawback ofcourse is that they will only fight with the "mother" division, not with the other corps/army units.
As far as I know it doesn't change the efficiency of the unit in attacking entrenched units.
"It happened, therefore it can happen again: this is the core of what we have to say. It can happen, and it can happen everywhere.”
¯ Primo Levi, writer, holocaust survivo