I just stumbled over this while doing some tests. Looks like you get better DR ( in theis case more texture in the shadows, while highlights stay the same) when using a Fader-ND filter (probably any ND Filter) than stopping down the iris.So here is a quick and dirty test
I used the MTF BMC and a Samjang 24mm.
Heliopand fader ND and a Heliopan IR filter on top of it.
I Stopped down the iris, till the 100% zebras on the bright spots in the background just went away.
I dialed in the fader ND, till the 100% zebras on the bright spots in the background just went away.
So in a real word scenario I would do exactly this, dialing it down, till my zebras just go away (maybe a even hair more, cause one channel may already clipping, without the zebras showing up). When using the aperture, my shadows are going much darker before I reach that level. Using the ND, gives me more wiggle room in my shadows.
Looks like the mystery is solved, why the ND fader does that trick:
As Gerardo Campos pointed out:
The reason is very simple: is because the ND fader are made of 2 polarized filters, so when you use a polarizer you cut light from the highlights giving you more space to play in your shadows; then the 2 polarized filters from the ND Fader in first place cut the highlights and when you start to rotate it start to act in all other lights.
And here is the reason why that doesn’t work with traditional ND filters:
the normal ND push down all lights equaly, so shadows and lights go down; but Circular Polirazed filter cut only hi light, is the best filter to obtain a blu sky and a bright terrain