YWT's are an exercise that look very easy.
They will work your upper back and rear shoulder muscles far more than you think for.
If bodyweight only becomes easy, hold a light dumbbell in each hand.
The key to getting the most from this exercise is to be squeezing the muscles when the arms are held in each position.
Don't just hold the arms there.... actively hold them there and make the muscles work!
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!