There are many times when we need to get our body into an unfamiliar position. The lack of familiarity could be due to novelty, or it could be a correction to an old, "bad" habit. A cue in this case is a trick to help achieve the new position.
I learned from Bobby McGee that a good arm position for endurance running is with the hands above the waist line, forearms roughly parallel and straight fore and aft. At no time should the elbows stick out.
My trainer, Sonya, taught me a neat trick to coax those elbows into position. Turn your hands palm up and gently tuck your thumb between your pinky and ring finger. Like magic your elbows will move it where they belong. I assume that after running this way for a time the thumb tuck will be unnecessary.
The other day at the pool I leveraged this tip to discover a cue for getting the correct catch position. Coach Susan Atkinson calls this 3D swimming -- "Drape, Define, Drive." Terry Laughlin describes it in this video. I was having trouble with the define step, where the elbow is lifted a bit as if gently squeezing a beach ball. A further opening of the armpit is another result of getting this step right. I was plunging my arm in after recovery, angled down as it should be, but trying to establish the hand position without lifting the elbow. My arm looked like a "Z." For the life of me I could not figure out how to lift my elbow. Suddenly I recalled Sonya's tip and focused on my hand rotation. It took a lap or two to work out that lowering the thumb and raising the pinky did the trick. For the right arm that would be twisting the wrist counter-clockwise. Not too much. At first, go ahead and start the pull with the hand twisted past the perpendicular, but once this starts to take hold in muscle memory lighten up so that the palm is perpendicular and pulling straight back.
I guess this works because our brain is habituated to paying close attention to what our hands are doing, more so than what our elbows are doing.
Keep in mind the kinetic chain. If you want to correct an action in one body part, look for a cause in a different but related body part. I know that my kicking leg action is entirely different -- my right leg stays straight and moves from the hip while my left leg bends at the knee -- so I expect to find imbalances between my left side and right side arm motion. Eventually I will need to sort out which is the cause and which is the effect. Maybe I will find a cue to help with that. Let me guess; it will have something to do with head position.