After reading all Yahoo site strings I believe I have deduced the movement rules regarding difficult terrain. I think this is correct.
"Woods and Marshes count as Difficult Terrain. If any base of a unit starts its Movement in a Wood or Marsh, the unit suffers a modifier of -1 from its Movement Roll. The unit must also pay the 3"/6" Movement Penalty for Difficult Terrain, as must any unit whose movement path enters a Wood or Marsh."
FROM this we can deduce:
1. A unit suffers -1 for movement dice throw if ANY base in the unit STARTS in difficult terrain.
2. A unit ALSO suffers the 3" /// 6 "' move deduction penalty if ANY base in the unit STARTS in difficult terrain.
3. BUT Once a unit moves, the unit pays the 3"/6" Movement Penalty for Difficult Terrain ONLY IF THE UNIT"S FCP (Fronf Center Point ) -- as opposed to individual stands in the unit-- movement path enters a Wood or Marsh.
4. Synthesis; IF ANY part of the unit starts in difficult terrain it suffers -1 on movement roll AND suffers 3"/6" deduction from the distance allowed. Once the unit is on the march no difficult terrain penalty is assessed for stands moving through difficult terrain, UNLESS the unit's FCP enters the difficult terrain.
Rewritten the Rule is thus:
"Woods and Marshes count as Difficult Terrain.
If any base of a unit starts its Movement in a Wood or Marsh, the unit suffers a modifier of -1 from its Movement Roll. Further, since any base of the unit started its Movement in a Wood or Marsh the unit must also pay the 3"/6" Movement Penalty for Difficult Terrain as it begins it move forward.
Any unit whose FCP movement path enters a Wood or Marsh after the unit commences its movement must pay the 3"/6" Movement Penalty for Difficult Terrain. However, if only a base of the unit enters the Wood or Marsh – but not the FCP path – the unit pays no movement penalty.
I suppose the justification for all this is that a unit beginning its move already fully deployed with some bases in the woods must pay a penalty as it traverses the woods.
However, when on the march the Division is not penalized for the woods unless "center mass" (the FCP ) is entangled in the branches and brambles. If only a part of the Division is at risk of "clipping" the woods as they march along, those fellows can avoid the woods and fall in behind or bunch up with the fellows in the center mass.
All this is illustrated in the following photos :
