The relative wind the propellor is flying in is related to the pitch angle of the plane. The downgoing blade sees a higher angle of attack, the upgoing a lesser. This creates more lift on the downgoing blade everything else being equal.
For single-engine planes it's a problem at high pitch angles.. and even is a major cause of the left turn when starting a takeoff roll, on taildraggers.
For twins, having both props turn "inward" so the down-going blade is closest to the fuselage eliminates the "critical engine" situation.
The P-38 was such that -both- engines were the "critical engine" depending on which stopped turning.
More modern planes have the inward turning configuration.