When using the theraband as resistance in arm movements it's helpful to keep external rotation of the upper arm bone - this way we strengthen the postural muscles and counteract our daily movements. Once I grab hold of a theraband I love to do roll up and teaser variations - the band gives us some support and makes an otherwise tough movement a little more accessible and a great way to access the skill of the movement while we build on the strength required to do the movement without the band.
Enjoy!