Perhaps the most vital component of effective and efficient throwing mechanics is the full extension of the throwing arm coordinated with the pronation of the wrist. Simply put, power is exerted through straight lines, or extended joints. This mechanic is called “triple extension” a term used in athletics and Olympic-based weight lifting. Think of poking someone with a pool noodle versus a 2×4. And as for creating trajectory, think of a gun barrel: the longer, straighter barrel creates a bullet with more force and accuracy because the pressure building behind it longer accelerates the bullet exponentially, and directs it on its path longer ensuring accuracy and velocity.
The arm works much like a trebuchet, a medieval catapult-type weapon that applies nearly identical physics. Notice the projectile releasing at maximum velocity when the sling is fully extended and in a straight line. The counterweight, lever, and sling, are identical to the mechanic of the shoulder, upper arm, and forearm/wrist.
The following video from ESPN’s Year of the Quarterback series where Sports Science investigates the physics behind 6’8″ Ryan Mallet’s 65 mph throw. (notice the 90 degree elbow position — though you’ll see Mallet drive elbow first, looping to this position, reducing movement economy)