As in CBT, knocking a mech down would generally cause injury to the pilot, potentially knocking them unconscious.

But yeah, once a mech loses a leg it is pretty much eliminated. It can prop itself up on one arm (assuming it still has both of them) and fire one arm at the enemy, but the torso weapons are generally unable to pivot into position to fire. The mech is an easy target and can only bring a small portion of its weapons to bear (unless it's a clan mech, then it's all arm mounted anyway.)

Realistically the legs are the guaranteed method of killing a walker. Something that large falling over would cause incredible damage to itself and the pilot would probably get a hell of a concussion from the impact. After that it is a sitting duck.

What I'm really tired of is players b*tching about how legging is cheap. What was cheap about it was MW4 had the damage transfer from a destroyed leg into the leg next to it, that isn't how it works in CBT, the damage transfers to the ajoined torso location, so blasting into someone's leg actually took 2-3 times the amount of firepower than simply blasting in the torso for a clean kill.

Of course, if you strip your leg armor and I fire a ppc shot at it and see it turn orange, you can bet I'm going to exploit your decision to get a few extra weapons or heat sinks.