I am not the original author, all credits go to lad09, he did make a lot of good mods for battletech you should check them out.
Lad09 was not online for a long time, I hope he is good and well.
I had problems to get the original mod working on Battletech 1.9.1 so I made a small fix. It is now working for me, maybe it will work for you, too.

I am using modtek for this mod and so should you.
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Lets Mechs knock down an opponent in one go, if they hit hard enough.

If an attack causes a Mech to experience stability damage 25% or more past
the enemy mech's limit, it will knock them down instantly instead of
just making them unsteady.

To give an example

Enemy Mech has 50% instability. They are not in unsteady mode.

If Mech A attacks them and deals out 60% worth of instability damage. This is only enough to make Enemy Mech unsteady.

If Mech B attacks them and deals out 90% worth of instability damage, this
will be enough to knockdown the enemy mech without needing to make the
enemy mech unsteady.

The exact instability limit varies depending on the Mech: an Atlas will be able to tank more instability damage vs
say, a Jenner.

TL;DR

See that yellow bar under mechs? If you max it out and then do like another bar's worth of damage, it'll
always be knocked down, even if the Mech wasn't unsteady.