This week's riddle can be found here. This week the blue ribbon goes to bsimon and hakomike!
The correct rules are that a unit may not attempt to rally if they are out of coherency. Also, as hako stated, a unit must fall directly back toward its edge each turn. That means that if you removed casualties from the middle of the unit and they break, your hosed! They never get a chance to rally.
Unless.... You herd your fleeing troops back together by blocking their exit with your vehicles or units. If something is in the way the fleeing troops move around. If done properly you're back in coherency.