Weak Zone of Control

[HerrDak]

Greetings.

Someone can explain to me the "weak zone of control" optional rule in Early American War series?

The Manual says:
"Select Weak Zone-Of-Control to allow units to move one hex per turn through enemy Zones-of-Control and to allow units to retreat from Melee through enemy Zones-of-Control."

Sometimes when I melee some enemy units they surrender and sometimes they retreat to an hexe under the zone of control of my units.

What Is the factor that allow the units to retreat or surrender?