It's about engine shielding, and minimum operating standards against emergency cutoffs.
Each critical slot doesn't represent underlying mechanics, but rather engine shielding, which prevents the fusion engine from escaping its containment bottle and stops the pilot from getting a massive dose of radiation. As the engine takes damage, the shielding starts to fail. After a certain degree of shielding loss (3+ critical slots), the emergency cutoffs kick in and the engine goes into a hard power lock.
Lower amounts of engine damage (1-2 critical slots worth of lost shielding) still result in significant waste heat spilling into the mech's inner workings.
Russ's line of reasoning isn't entirely correct, but the spirit of the idea is true, and the end result is pretty much accurate to what it should be (especially if it's just a starting point, pending balance adjustments).
Edit:
My preference would be for it to generate extra heat, rather than reduce heat dissipation rates. +0.60 heat per second would be about right as a starting point, based on losing 2 engine crits, IIRC.
Edited by Levi Porphyrogenitus, 29 September 2014 - 09:40 PM.