When does a unit go "under" an entrenchment?

[plloyd1010]

To be clear, it is these guys:


Holding off these guys (in this order):


While it doesn't take long to jump into an entrenchment, it takes time to move through entrenchments and deploy in entrenchments. RaW does say they automatically get the entrenchment benefit, it says they have to choose it. The question is, when can/is that choice made, considering the word occupy is part of the equation?

BtW: We are play pretty RaW. The only house rule is that flanking fire is not applied against units in reg wood, towns, blockhouses or forts.