Why have a relaxed bow arm?

A locked elbow acts like a solid linkage that forces the arm to rotate about the shoulder.  It takes 12 to 16 millisec. for the arrow to leave the bow after the string has been released.  Any lateral movement during this time will deflect the arrow.  Bearing in mind that the reaction to releasing the string is automatic, the forces and torques existing just before release persist after release and force the bow to move under their influence.  If they are always the same no harm is done adjusting the sight will compensate but with fatigue the shoulder tends to move up or down so that when the arrow is released the bow moves laterally a different amount every time leading to dispersion of the arrows.

Some good archers appear to have their elbows locked at full draw (e.g. Darrell Pace - FITA world record holder for many years).  On close examination the natural position for their locked elbows results in a bent arm.  Under the tension of the bow the arm acts like a stiff spring.  When the string is released the spring "uncoils" and the bow moves forward directly towards the target.  The archer whose arm does not work like this it is best to introduce some flexibility at the shoulder, elbow or wrist.

An underlying assumption in the note on stabilisation is that the bow arm is relaxed in the way described above.