Whenever a BattleMech suffers an amount of damage beyond a certain threshold within a short duration, the amount of damage that would be dealt above that threshold is reduced.
Shooting at someone with all your guns will having diminishing returns over cycling them over the course of a few seconds, but if you need to get that snap shot in before the target disappears behind a hill or a building you still can - but you won't obliterate them.
It won't make the big guns do any less damage individually, snipers will still be able to lay down pain on brawlers as they close in, but a heavy or assault BattleMech will be tough enough to give the MechWarrior inside enough confidence to take that alpha from the Awesome or Stalker.
For example, let's say that the threshold is around 20 damage. You can hit someone with a bunch of medium lasers, or a couple PPC's without suffering any loss of damage due to the threshold. Fire more than that within half a second and you'll find that they'll take less damage than they use to, perhaps much less than if you spaced your shots out - but still more than if you hadn't fired those extra weapons (because you died or something blocked your line of sight).
Choosing to fire an alpha that will shut you down will become less of an automatic decision, will the reduced damage be enough to finish it off? If it is, then it might be worth it, but if not...
A knock on effect would be that focusing fire, while still more deadly, would also begin to suffer from diminishing returns. It would be more efficient for a lance to spread it's fire between two or three targets, leading to longer battles where everyone feels a bit tougher.
Lights would probably be the least effected, since they only ever tend to take one or two hits at a time - but they're also the least likely to lose much damage from the change.
Mediums would probably benefit the most - they're less likely to lose much of their damage, but they're generally slow enough that most 4-6 PPC snipers can peg them in the centre or side torso with relative ease.
Heavy and assaults would probably suffer the most - they'd lose damage to the threshold more frequently and in greater amounts, but they'd become much, much more durable.
If people need a flavour and cinematic feel for their rules, you could look upon the threshold as the point where some of the shots or beams start punching through the same hole, exiting on the far side of the 'Mech without really hitting anything that the first part of the salvo hasn't already reduced to a molten pile of slag.
Edited by Gretik, 11 July 2013 - 02:37 PM.