Not sure if I have the name right - it's either a displacement field or a displacer field.
This isn't the armour-energy-field thing, but something that's popped up in some Horus Heresy short stories - one used by the Custodes in "blood games" and by an ultramarine in the Iron Warriors short story with the upside-down fortress.
Essentially, it shrinks you down to disguise the bulk of a post-human, which I figure opens up a lot of options for nefarious doings in deathwatch missions.
I'm trying to find out if there are any more mentions of this device, especially its limitations, with the ultimate aim of writing house rules for it.
The things I can think of being a problem:
1) Astartes are supposed to have a distinct flat-featured look which Custodes may not. Not everyone would recognise it, of course, but particularly educated opponents might.
2) There would be a distinctive way astartes would move, even out of armour, because they never flex their back between shoulder blades and the small of their back due to the whole "fuzed rib-cage" thing
3) Not sure how well it holds up under physical contact - I can't remember if the custodes in blood games actually shake hands with anyone, for example.