The first time that I was introduced to this movement pattern, in testing, I was confused and flustered at how challenging this was and at how relevant it is when we are usually trained to flex the opposite shoulder to the extended leg when in some of our exercises.
With that being said, this perception changed. The movements that are used in the functional movement screen are there to shed some light on instabilities and asymmetries. That means that every movement that you perform in there is made to make it easy for the evaluator to see if you have dysfunctional movement patterns.
This last movement, although not the least, challenges the stability in multiple planes of the pelvis, core and shoulder girdle when the athlete is moving both the upper and lower extremities. What that means is that it evaluates how you are stable in a three-dimensional way.
Logically, you may wonder how can this be of value for you to know if you can or cannot do it? First off, when you cannot perform this movement, this may mean that you do not have an optimal reflex of the stabilization of your core or maybe even that you have poor scapular and hip stability or this may also be related to pour mobility as well. It is also important to know that if you cannot clear an upward dog/cobra position without pain after the test that you cannot pass the test. You need to further investigate on what is causing the pain.
My question for you is how are you at rock climbing? This kind of effort in rotary stability is definitely observed in that sport. Although a lot about rock climbing is also related to gripping strength, poor core stabilization will limit your ability to transfer force from lower body to upper body and vice-versa, which means that you waste energy and that you cannot climb as long as you wish to climb.
Interesting now isn’t it? Now how can you work on that core strength? You can start by working on your planks. If the plank is an easy task for you, you can try to extend a leg while doing it or a shoulder or even both at the same time (diagonal planks and yes, this will challenge your rotary stability). What is important here is that you respect your body and that you commit to working on it. The length of your workout is not necessarily what is important. What is important is how regular are you at committing to your workout or more precisely, committing to yourself?