Stick. is referring to clockwise rotating donks so the L. becomes the critical engine.
The critical engine is the one that would have the greatest adverse effect if it fails.
With clockwise rotation (viewed from behind the a/c) the overall thrust line each engine produces is offset slightly to the right. This is due to the same reasons as a single engine a/c's yaw tendency with power application.
For the LH engine, this offset moves its thrust line closer to the longitudinal axis. For the RH engine the thrust line moves further away from the longitudinal axis.
The further the thrust line is from the longitudinal the more inefficient flight will be when asymmetric due to greater slideslip effects & additional control surface deflections causing drag.
If the LH engine fails (or is deliberately stopped
) then the a/c is left with the RH engine's relatively greater offset thrust line adverse effects. If the RH engine fails, the a/c is left with the relatively lesser adverse effects of the still running LH engine's offset thrust line.
Since the adverse effects are worse if the LH engine fails (for clockwise props.) the LH engine is the critical engine.