I was taught that you don' t want a straight line from bow hand to release elbow while at full draw. Almost straight, but not quite. Reasoning is, if you already have that straight line across your back, and then you release, the natural movement of your follow thru will cause you to pull your bowhand to the left (right handed shooters) at the shot due to all the tension. If you keep it just short of a straight line, when you shoot, then the follow thru will put you in that straight line all the way from elbow to target and there is far less bow hand movement at the shot.

Made sense to me at the time.