Last week there was some discussion in class about "extending the trunk out" over the front leg in Utthita Trikonasana as opposed to just "reaching the hand down".
Trikonasana is the first pose where you learn to extend the trunk out over the front leg. Both sides of the waist should be equally long.
In this version of Utthita Parsva Konasana (bottom arm inside the front leg, top arm straight up) see how the trunk is lengthened out over the thigh so that the knee and armpit are in contact with each other. This length and connection is important to many poses.
Here is that same relationship of the side of the trunk along the length of the thigh in preparation for Marichyasana I
The trunk has to be elongated to come into the final version of Marichyasana I.
We worked on this in class. Here is the link to rent the video.