ORIGINAL: Silvanski
note that HQ's may have trouble disengaging by themselves depending on enemy recon, terrain, movement points left etc
Provided the hex they move to is friendly occupied, they get to disengage for free. It is correct that this consumes a lot of MPs, so the HQs need to be high mobility. Artillery units get the same privilege, and can be used as above, too. I think this was intended to model the fact that such units are rear-area elements, and, as such, would have some advance notice that the enemy was on his way - allowing them to clear out ahead of the spearheads. So, this compensates for one of IGOUGO's issues.
The above would then be an abuse of the privilege - but it does work. One day, it may get reined in somewhat (see item 7.6 in the wishlist).
Another way to disengage free is to use limited attacks. Obviously, you have to retreat all adjacent enemy. But, if they're weak enough, it allows everyone to escape.
One more is to save your most powerful, healthiest, and highest MP unit to be the last to disengage - and hope that it has the ability to not get stuck. Again, this is only if the enemy are relatively weak or foot bound.
Last, of course, is to subdivide a weak unit and leave it behind. Of course, there are those that consider this to be an abuse, too. (See item 7.5 in the wishlist). So one day, even this may require some consideration of just how large a unit to abandon.