Pitching Instruction footnotes
This is when the body rotation and forward momentum are so far ahead of the arm that the ball is forced behind the shoulder until and even after heel-strike. This causes the arm to "whip" and have to "catch-up" with the body. This is a very bad, and common, habit of youth pitchers. It places a lot of stress on the front part of the shoulder and the inside part of the elbow, and leads to pain and injury. It also causes poor right-left (inside-outside) control. This is a habit that must be corrected early (before age 12).
Technically, extension beyond zero of the abducted glenohumeral joint. What this means is the throwing arm reaches behind the back, and the catcher and batter can see the ball behind the head when the pitcher has reached back and is cocked. It must be avoided as it over-stresses the shoulder and elbow. It goes hand-in-hand with catch-up, above.