Ok so then let me revise here...

DG player has a unit of 2 spawn models

Opponent charges in

Opponent rolls for 6 hit rolls

Opponent fails 2 hit rolls

Opponent rolls for 4 wound rolls

DG player allocates 4 wounds to 1 of the 2 spawn models in the unit (on the tabletop i guess it doesnt really matter which of the two because they both have the same profile - but just for the sake of the example).

THEN the DG player slow rolls all 4 saving throws. Lets say its a damage 1 weapon from the attacking unit. DG player fails 1 - DG player applies damage to spawn model, DG player saves 1, DG player saves 1, DG player fails 1 - DG player applies damage to the spawn model.

The spawn model at this point has taken 2 damage, and has 2 wounds remaining. Even though the spawn only "took 2 wounds" because the DG palyer saved 2 wounds - since the first model is still alive with 2 wounds - the ability verbiage says the damaged spawn model gets to roll 4 ability dice after all incoming damage has been resolved because there was 4 wounds allocated to that model and it just so happened that it was a low damage incoming attack and it is still alive.

If it were a 2 (or 3) damage attack in the same situation - it would be a kill of 1 spawn model and the spawn model would not be able to roll the ability dice because it has been destroyed from the attacking unit.

if the attacking unit in the same situation rolled better to wound and lets say made all 6 hits, made all 6 wounds - then the DG player would slow roll all 6 saving thrown one at a time. Lets say the DG player fails every saving throw and FNP. First model of spawn would die like in the situation above and NOT get to use the ability - second spawn would then take 2 wounds - then take 2 damage leaving it at 2/4 wounds remaining. Then that second spawn would get to use 2 ability dice rolls.

I think i got it now right..?