Whenever a weapon like that (PPC, AC/20, AC/10, Gauss Rifle) removes the last bits of armor from a part of the mech the critical strike delivers the full damage of the shot in question to the components in that part.
What this means is that if a mech had 19 points of armor left on a location and it gets hit by an AC/20 round, the bullet in question will surely take out any component it gets a critical, despite the fact that it barely scratched the internals.
Sadly my PC is not good enough to record this on video while playing the game on a reasonable FPS, but it can be replicated in the Training grounds as far as i know.
Edited by Bloody Moon, 11 June 2013 - 03:28 PM.