Whenever a BattleMech suffers an amount of damage above and beyond a certain threshold within a short duration, the amount of damage dealt above that threshold is reduced.
Shooting at someone with all of your weapons will result in diminishing returns, it would be more efficient to fire them by cycling through them over the course of a few seconds. However, if you need to get that snap shot in before the target disappears behind a hill or a building you still can - but your alpha won't obliterate them.
This change wouldn't make any of the big guns do any less damage on their own, snipers will still be able to lay down pain on brawlers as they close in, but a heavy or assault BattleMech will be durable enough to reach the brawl (assuming he torso twists, uses cover and isn't the only one charging).
For example, let's say that the threshold is around 20 damage. If you hit someone with a bunch of medium lasers, or a couple PPC's, you won't suffer the loss of damage due to the threshold. Fire more than that within half a second at a single target and you'll find that you will have dealt less damage than you would if you had spaced your shots out - but still more than if you hadn't fired those extra weapons at all.
Choosing to fire an alpha that will shut you down would become less of an automatic decision. You would have to ask yourself whether or not the reduced damage be enough to finish your target off?
A knock on effect would be that focusing fire, would also begin to suffer from diminishing returns. This would mean that a BattleMech that pops its head up is less likely to be instantly annihilated by a lance of snipers, but the short duration would mean that in a whirling melee or turkey shoot, where your volleys will land in a less consistent fashion, focusing fire would still be worth doing.
But how would this affect each weight class individually?
Light mechs would be the least effected, since they only ever tend to take one or two hits at a time. The reduced damage would mean that a full on alpha from an assault might not blow a leg off, but it still wouldn't be a healthy experience.
Medium mechs would probably benefit the most out of the change. They are unlikely to be doing enough damage to be regularly going over the threshold, while they are often slow enough that it is fairly easy for a skilled sniper to peg them with an entire alpha.
Heavy and assault mechs would probably feel the changes the most heavily. Their weapons would feel alot less powerful when fired in an alpha, but their ability to survive would be drastically increased!
For those people who need to have a way of visualizing mechanics for them to make sense, the damage threshold could be seen as the point at which the rest of the projectiles or energy from the volley is simply punching clean through the holes made by the first part. Some of the projectiles might clip the edges of the hole, or some of the energy of the beam would be dumped into the cloud of plasma boiling away inside the victim's BattleMech,