FFMO:
Moving about in an area without cover makes you easier to be hit by bullets and shrapnel.
FFNAM:
Moving about upright rather than crawling presents a larger area that can be hit by bullets and shrapnel.
Pretty easy to understand the concepts behind FFMO/FFNAM IMHO.
If you're being targeted by infantry fire, sure. But if you're firing a Gun or mortar, it's less clear.
The German 4-6-7 is standing motionless in Open Ground when it's attacked by the Russian mortar. The 8-3-8 is running across Open Ground ground when it is attacked.
Why is it so much easier
to hit the 8-3-8 than it is the 4-6-7? The mortar (or Gun) has to track a moving target, surely that would be harder than hitting a stationary target.
Once you've been hit, then the effect would be more lethal (certainly for FFMO), but you would think that being hit in the first place should be more difficult.
I think you're talking about "moving about in an area
without cover" vs. "moving about in an area
with cover" and "moving about upright rather than crawling", whereas I'm talking about moving vs. not moving at all. And the difference between being attacked by infantry fire, where FFMO/FFNAM applies to the effect DR, but for Guns and mortars, it applies to the TH DR.
You might say that the mortar is attacking the entire hex, and so the 8-3-8 has to run a gauntlet of shells, making it more likely to be hit, which is maybe possible, but replace the mortar with a 150mm Gun. That argument surely doesn't apply any more, but it still gets a -2 TH DRM. And in any case, however many rounds the Gun or mortar might be able to get off, it would be the same in the attack on the 4-6-7, and in all likelihood, more of them would be on-target since the poor suckers aren't moving