Though I agree that shields probably shouldn't prevent hackdown hits, the current wording of the rules strongly indicates that shields should be useful
I think it's important to quote the relevant parts of 1.12.7 in its full context:
Quote:
In addition, the losing formation suffers a number of extra hits equal to the difference between the two sides’ result scores. There are no saves for these hits, which represent units being hacked down as they turn and run, or disintegrating into a panic-driven rout as they flee. Remove these additional casualties as you would hits inflicted in the assault phase (i.e., units in base contact first, then those closest to the enemy, etc.).
So:
-Hackdowns are applied as hits with no save
-The reference to units suffering these hits as casualties later on is a bit premature, but understandable, as at the time of the core rules being written none of the trouble units I've found here existed. I think reading this as 'Apply these hits as you would hits inflicted in the assault phase (i.e., units in base contact first, then those closest to the enemy, etc.)' is what is intended.
From this:
-As others have pointed out, all hits would be allocated to what is on the board- namely the transports. These units would then make their saves (which are none), causing their transported troops to come out (and forcing saves on them). As all hits of the one kind must be allocated before making saves, and hits can't be allocated to the transported troops not present on the board, these transported troops are immune to the direct effects of the hackdown, even if the hackdown hits would cause overkill of the transports (some transports would die twice, to no purpose). This is the same as hit allocation during the combat and shooting phases.
-A War Engine would take as many hits as required to destroy, not die to a single hackdown hit (crit rolls would be made as normal). This may be a bit of a rarity, but could be vital- I originally started along this whole line of thought when considering how to take down Orkeosaurus, which Feral Orks can spam (as happened in a recent Australian tournament).
-A Fearless transport would perfectly shield any troops inside from hackdown, in the same way an armoured vehicle perfectly shields any transported infantry from AP fire- it simply can't take the hit. How this then works for things like the Cadian Stormlord (where transported troops can actively FF) is a confusing matter for that particular list. It could be a serious matter for other lists, depending on how 'transported units may shoot' is interpreted (does this allow FF?).
-Shields are still an issue. Being hits, there is no obvious reason why shields shouldn't absorb the hackdown hits in the same way as any other hits (though things like Eldar Holofields, which give a save, are useless).
--This then raises the usual issue of hit distribution, shields and DC. In a formation with two Barges of Pleasure, for example, by the current rules as written you'd distribute 3 hits to the first Barge then 3 hits to the next Barge before returning to allocate more hits to the first Barge. As they have 2 shields each (and assuming these are still up) each barge would therefore only lose 1DC from 6 hackdown hits (plus all shields down). That's obviously a very different outcome to if shields didn't prevent hackdown.
--Hackdown hits are distributed 'as hits in the assault phase', but not specified as FF or CC hits. This can be relevant to purposes of ignoring shields, as most shields don't work for CC but do for FF.
@Dptdexys-
My initial question came from reading the printed rulebook- I am now reading the online rulebook, which does fix this problem.
As to the matter of 'threat' definition, ability to shoot is not a good guide here- the unit to which the line is drawn doesn't have to be armed with any ranged weapons, not even small arms. Fortunately the issue is made mostly clear in the online rulebook.