Why use a bow-sling?

When the arrow is released the only contact between the archer and the bow is the front hand.  The initial poundage drops rapidly to zero and the natural reaction is to grab the bow to prevent it falling.  Any asymmetry in this action causes a torque to be applied to the bow.  If an arrow is still on the string at this time it will be deflected from its intended course.  The worst case scenario is for the archer to anticipate the release so starting the deflection even before the arrow is released.

By using a bow-sling the need to grab the bow is eliminated and with it any excuse for anticipating the release.