Partial retreats optional rule

[Embis]

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?