The main principle you should be looking into is P-factor. If you lose the left engine you have to pitch up to substitute for the loss of lift to maintain altitude. When pitching up you are increasing the the angle of attack of the descending blade, which creates more of a "bite". The arm is longer from the longitudinal axis to the descending blade on the right engine and shorter on the descending blade of the left engine. This increase in P-factor and longer arm on the right side is what makes the left engine critical whenever both propellers spin clockwise. There are three other factors that contribute to the left engine being the most critical to lose, but P-Factor is the main one to conecentrate on. Hope this Helps!