When a detachment becomes Broken, remove its order marker (if any) and place a Broken condition marker.
A detachment becomes Broken when it loses a close combat or a firefight.
When a detachment first becomes Broken, you must make an immediate Retreat move with it. Thereafter, apply the following rules while the detachment remains subject to the Broken condition:
You cannot give orders to your Broken detachment — so it can’t shoot or enter overwatch, and it can’t move in the normal way either. [TODO: Can a detachment still Snap Fire while Broken?]
You may make a Retreat move in the Movement phase, if you wish. This is the only type of move you may make with a Broken detachment.
If the enemy engages your Broken detachment in close combat or a firefight then you must halve your detachment’s Assault and Firepower totals in the engagement.
The Retreat move is a single move in which you may move all mobile units in the Broken detachment up to 20 cm — regardless of their actual Speed values.
You may use the Retreat move to move the units in any direction, subject to the normal rules for terrain effects on movement.
You may 'move' units 0 cm if you wish. Or, it may be that you can’t move some units anyway. 
If you end a Retreat move such that any Broken units are within 15 cm of any enemy unit  , then they are destroyed — remove those units as casualties. (They are killed by the advancing enemy or scattered in the confusion — they play no further part in the battle.)
The Broken condition and the 'out of command' rules
If a Broken detachment has units that are out of command (that is, more than 30 cm away from their detachment HQ unit) then those units are not affected — they are not Broken and the out of command rules still apply to them.
If units that are out of command become Broken, remove them as casualties immediately. There are no other effects on the parent detachment. Similarly, if units in a Broken detachment become out of command, remove them as casualties immediately.
Rallying a Broken detachment
You may be able to rally Broken detachments in the End phase.