There has been some confusion on what the erratum for Defensive Formation (DF) means.
What this erratum does is move the DF from AFTER the activation to IN the activation. As a consequence, only the models that have not yet ended their activation can perform the move. This brings DF and Reform to the same mechanism.Replace the second sentence of Defensive Formation with the
Immediately after models in this unit complete their actions this
turn, models in this unit other than this one can make a full
advance, any previous orders this unit was issued expire, and this
unit is affected by the Shield Wall order.
A running model can't "complete its action". If you run you do not have an action. If you do not have one, you can't take it nor forfeit it, so you can't complete it. Same would happen to a model which fails a charge.
If at least one model in the unit takes or forfeits its action, the other models may use DF (except those who ran or failed a charge or anything similar). This last sentence is telling that you don't need to have each and every trooper completing or forfeiting an action to trigger DF.
The movements for DF happen after all the actions are completed, so it is:
action model A
action model B
action model C
and THEN the DF moves for A, B and C.
Unit with troopers A, B and officer C who uses DF.
All make a full advance and then make their action. Then they all get to make their DF move, except the officer. All are affected by Shield Wall.
All run. None of them gets to make their DF move and none are affected by Shield Wall as no one in the unit completed its action and thus never triggered DF.
They get the charge order. A runs, B fails the charge, C makes a successful charge. No one gets to move as the only model that has not yet ended its activation is
the officer who is not allowed to move. But all are affected by Shield Wall.
A is knocked down and forfeits action to stand up. They get the charge order.
Due to this
A makes a full advance and then ends its activation. B fails the charge, C runs. None of them gets to move, none are affected by Shield Wall.Originally Posted by Prime, p. 46
The same basic principles apply to the Reform ability (Kossites, Striders with UA, etc.).