The last bullet point on p. 258 TW states, that under Forced Withdrawal rules "Vehicles and 'Mechs that are immobilized - usually through motive, leg or gyro damage - are shot down and typically abandoned[...]". There are two problems with that statement.
Problem 1. It uses word "immobilized" without defining what it means in game terms. It certainly does not mean the same as "immobile" as described on p. 110 under "Firing at Immobile Targets", since no amount of gyro damage can make a 'Mech an immobile target, and the only way to make a 'Mech immobile through leg damage is to destroy all four legs of a Quad. At the same time reactor shot-downs due to heat or pilot/crew unconsciousness should be no reason to abandon a unit (either because it will likely be operational during the next turn in case of a shot-down or because the pilot/crew is in no shape to leave in case of unconsciousness). Immobilization for the purposes of Forced Withdrawal should be better defined then. In particular it should be answered if:
- a vehicle, that has been permanently reduced to 0 MP by damage, but not immobile is considered immobilized for the purposes of forced withdrawal,
- how much leg damage a 'Mech needs to suffer to be considered immobilized for the same purposes? In particular is a 'Mech considered immobilized if it loses one leg (two in case of a Quad) and is reduced to 1 MP, and needs a very difficult PSR just to get up (making it unlikely to leave the field without tearing itself, or it's pilot apart in the process, much less continue walking around and fighting; remaining on the ground an shooting from there also seems like a desperation move if you can't reliably stand up before some enemy 'Mech runs over to you, unloads all its weapons with a -2 to hit for firing at a prone adjacent 'Mech, and kicks you for good measure), or it is considered immobilized only after being permanently reduced to zero MP by loss of two legs (three in case of a Quad)? As far as the fluff goes the MechWarriors in the stories I've read seem to abandon their Mechs after losing only one leg, but I don't know if it should translate directly to the game.
In case of gyro damage it seems obvious that this rule kicks in only after gyro destruction, since in the first bullet point of the Crippling Damage section it clearly says that a 'Mech that has suffered one gyro hit need to also suffer one engine hit to fall under Forced Withdrawal rules, and all gyros under TW rules are destroyed after taking a second hit. Still it would be nice to see it explicitly stated in the rules.
Problem 2. None of the rules from the last bullet point on p. 258 TW (abandoning immobilized units) appear in the BMM (on p. 81 where Forced Withdrawl is described). The only rules there are from the first bullet point on p. 258 TW (crippling a 'Mech). The only thing about 'Mech being immobilized is that under Forced Withdrawal rules it is considered destroyed if it can both no longer move AND has lost all of its weapons. the rules from the last bullet point, that are relevant to 'Mechs, should probably be added to BMM (after they are clarified as described above).
My take on solving Problem 1: Personally I would make crews abandon their non-infantry units during the nearest End Phase (and considered destroyed for the purposes of TW/BMM):
- if they are permanently reduced to 0 MP (in case of grounded Aerospace units maybe with an exception of DropShips if they were made permanently unable to lift off),
- if they were otherwise permanently unable to leave their hex (like a vehicle stuck in a double basement - see p. 179 TW), unless they were deliberately deployed in such place at the beginning of a game (for example a vehicle or a 'Mech with no jump/VTOL/WiGE movement deployed on top of a tall one hex building or hill because of the superior LOS, or protection from rampaging Berserker or Neanderthal 'Mechs such place may offer for example); of course the players can always rule (and under typical circumstances probably should), that no such deployment is legal,
- in case of 'Mechs if they lost their gyro,
- in case of 'Mechs if they lost at least one leg (two in case of Quads), as they would need to make a +5 PSR roll just to stand up (one in six chance for a regular skill 5 pilot, difficult even for veterans), and another +5 PSR roll to avoid pilot damage if they fail the first one, and they would be limited to one attempt per turn, and if after standing up they were for any reason forced to make another PSR for any reason they would likely be back on the ground with an even more banged up pilot.
A side benefit of this solution would be that you could scratch the bit about a unit being considered destroyed if it can no longer move and has lost all of it's weapons (both from the first bullet point on p. 258 TW, and from p. 81 BMM), as just being unable to move would be enough to force crew to abandon a unit, and that would mean that a unit is essentially destroyed.