Resistance exercises using tubing were first made popular by Dr. Frank Jobe as part of his rehab program following injury or surgery. He said they should be performed 3-4 times a week as part of a warm-up before weight training. Of course, he also explained them using clinical terms such as abduction and aduction, internal vs external rotation, flexion, etc. Nothing wrong with that, but in working with ballplayers at anything below college age — unless there's a school test with marks involved — it's best to skip the technical talk and just show them what to do and the proper way to do it. (For those interested in the Jobe details, click here.)