I am bringing the quote below over from a recent thread on another site. If you suffer from plucking the string, and have been coached by someone such as Rod Jenkins or Arne Moe in proper form, but still may be having issues, I hope the quote below from "Greysides" will be of help to you. Many of us who suffer from this were using the shoulder joint as our pivot and bringing the elbow around, thus the pluck.
"The difference in some cases is that instead of the shoulder joint being the pivot point and resulting in plucking, the back muscles should pull on the combined scapula/shoulder joint/arm unit.
Consequently, instead of the arm rotating from the shoulder with the shoulder socket and scapula staying more-or-less static, the whole shoulder is pulled back and takes the arm with it."