In years past, a bright mind by the name of Isaac Newton observed a few key occurrences in nature, eventually calling these Laws of Motion. Aircraft, much like the rest of the world, are not special when it comes to these basic laws. The most notable effect here pertains to Newton's Third Law - every action has an equal and opposite reaction. 

​The Torque Effect occurs when the rotation of the engine rotates the propeller - and, as a result, the entire aircraft must experience a counterclockwise moment (all propeller aircraft in the U.S have clockwise spinning propellers, viewed from the cockpit). 

The reason that this effect is contributed to the 'Left Turning Tendencies' is due to the fact that, unimpeded, a roll induced in the aircraft will inherently induce a turn. 

Torque Effect
https://cdn.boldmethod.com/images/learn-to-fly/aerodynamics/why-you-need-right-rudder-on-takeoff/engine-torque-effect.jpg

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