The Lakṣana of Parśva Trikoṇāsana, or side triangle pose,
is as a movement OF the spine to the side over one leg,
rather than as a bending or arcing IN the spine towards the side.
Thus the aim is for the spine to stay straight relative to the leg,
with the intention of extending it from crown to coccyx.

Link to Series: 108 Postural Practice Pointers