Partial retreats optional rule

[Steel God]

As a newbie to this era I am struggling to understand what effect this rule has, or what effect it doesn't have. I think the manual gives a rather unclear explanation, but maybe I just need to have another cup of coffee, can anyone clarify?

From the manual:
"Select Partial Retreats to allow some units to retreat from a hex after being defeated in melee even when there is not enough room for all of the units to retreat. See the Design Notes in the User Manual for why this is not a default"

and the user manual design notes it says: 

...The dynamics of this situation is known as "choking" and results in greatly reduced
flow through openings. Based on this, there is justification for concluding that
obstructions to their retreat would often result in the surrender of the defeated
troops rather than some optimal reduction.



So does selecting this optional rule make it less likely that a unit unable to retreat due to over-stacking would  be eliminated, or more likely?

One man's interpretation, I reserve the right to be wrong.

Selecting Partial Retreats should result in less eliminations.  If a retreat would cause over stacking the entire unit would be eliminated without this rule in play.  With this rule in play, the unit would retreat, and be reduced to the level necessary to not create over stacking.

Naturally the ability of a unit to retreat in any given situation assumes there is a place to retreat to and that is in turn impacted by normal or weak ZOC rules.

That is how it would work in ACW.  In Musket & Pike there is an additional wrinkle of an optional rule "No Melee Eliminations" which means units will not break and surrender if they over stack, but will stay in place and be reduced in a manner like PzC assaults, and you would have to melee over a few turns to completely batter a unit down.