I am posting this BI question here as it is also a Impetus question.
I am just reading the new version of BI to identify differences between the new and old versions. I think I have been playing movement in terrain wrong!! I have always used proportional movement.
Example:
A unit can move 8U, it faces a wood 2U away. I have always moved the unit 2U in the open and 3U at half speed in the wood.

However after rereading BI2008 and checking in the Impetus rules as well, about difficult terrain. which says
"Leaving this ground does not lead to disorder, but you must move at half-speed."

Does this mean that in a move which a unit is in terrain for part of it move, is at half speed for all its move?
Example:
A unit can move 8U, it faces a wood 2U away. Should move a total of 4U due to the difficult terrain.

Neither BI or Impetus specifies how this works. I know the wording has not changed but I was just reading very carefully this time.

You made correctly.
You have to use proportional movement.
The only exception is when you leave a difficult terrain. You are not disordered but simply you move at half speed, doesn't matter the actual distance you perform in the difficult terrain.
It is like you loose more time as you automatically reorder. This rule is to allow also ambush or similar action (imagine a rush from a steep hill).

Are the D6 dictated charge bonus and retreat distances also halved in difficult terrain? Eg 1/4 D6 for infantry and 1/2 D6 for mounted?

Thanks,

Simon

if you charge in difficult terrain you calculate the total before and then halve where required.
Example. Unit A (FL with M= wants to charge B that is in a difficult terrain and 7U away. A is 2U out of the terrain, so has to make other 5U in difficult terrain.
Without the charging bonus FL could move further 3U in terrain so cannot reach B. A decides to use the die roll and rolls a 5.
Ok, the total movement of A is 8+3 (5/2 rounded up) =11. 2U are used to move in plain terrain the other 9U available to move in difficult terrain that is A can move further 4.5U and cannot reach the enemy.

Of course this is just to explain the case. For a FL its is better to make 2 movement phases!!!