This clip has been around for a while now, but it is still the best example of the result of what can happen when a low kick lands dead centre onto a defending knee:
and it has happened in the cage as well:
a more detailed example including discussion and slo mo:
check the reply at 2:16 - the kick is thrown and the defender shields.
the kicking leg lands dead centre of the shin and bends and snaps
the shielding leg takes it at the top.
if a kick lands with the "knuckle" at the end of the bone above the foot it will be powerful -
if it lands dead centre of the shin then the bone will snap like a twig
Useful to know when both attacking and defending with low kix.