These are called "Standard Engines". XL Engines (aka Extreme Liability engines) halved the engine weight (good!) but also put extra engine criticals into the side torsos (2 for clan, 3 for IS.) This meant that with the destruction of a side torso (very probably when you mount things like Gauss Rifles or weapons with ammunition there) then you lose those engine crits. This spells instant engine destruction for Inner Sphere mechs, and 2/3 engine hits for a clan mech, as well as +5 heat per engine hit/turn.
The rule for catastrophic engine explosion I am most familiar with is from the CBT Tactical Handbook which states (paraphrased): Any mech that has all of its center torso internal structure destroyed in only a single turn has a chance of catastrophic engine explosion. Roll 2d6, on a 8+ the mech's fusion reactor explodes. Anything in the hex where the mech exploded is instantly destroyed. Anything within 30 meters takes damage equal to the destroyed mech's engine rating divided by 5 in 5pt damage clusters (yes, this means a Timber Wolf going crit with it's 375 engine is doing 75 damage to everyone right next to it.) Anything 31-60 meters away takes engine rating divided by 10 in 5 pt clusters, and between 61-90 meters away they take engine rating divided by 20 in 5pt clusters.
It was pretty rare because weapon hits would usually scatter over the target, and one hit weapon kills were pretty rare, they'd usually blast off all the armor and dig deep into the internals on one turn, and then the mech would get finished off the next.
