Imagining the correct posture will not physically mould your
body into the “ideal position” no matter how long you think about it.
So the simple answer is, yes, you do exert a light rotation of the shoulder
muscles to allow both the index finger and the thumb to rest against the
thighs, so long as this turning does not go so far as to force open the body.
Once the correct posture is established (to the best of your
current ability), you then want to relax your muscles groups while maintaining
the posture. You initially use sinking qi down through the arms and out of the
fingertips (and later dissolving) to clear from the body whatever tensions are
pulling back the shoulders. It is the tension in the shoulders that causes the
rotation in the arms and prevents you from achieving the ideal posture.
The purpose of standing practice is to make soft adjustments
to the body in order to access and release accumulated tensions. As these
tensions are released, the body will naturally align without effort or use of
any muscular force, enabling blood and qi to circulate more strongly. This will
in turn allows you to access deeper layers of tension. Throughout the whole
process, you are working towards your core, releasing ever-deeper layers of
tension and, as you do, generating health and vitality from within.