[neil]

You want your left arm to be as lifeless as piece of rope. The left arm changes conditions (to becoming taught, or straight) because of extensor action which is produced by action of the right hand. Left arm being or becoming straight is not a function of the left arm (rather a fuction of extensor action).

You can keep the left arm straight without extensor action-and I would not call it an action of the right hand-more right triceps .However I'm not splitting hairs -extensor action is important BUT some people already have it -and don't know it