Put simply, this is the best Pec. minor, Pec. major, biceps stretch we have been able to design, so far. In addition to the muscles (and, arguably, of more importance), getting in to the final position stretches the entire fascia superficial and deep front arm line – this extends from the mid-line of the chest through to the palmar side of the forearm, down to the tips of the fingers.
The exercise is shown in a one-partner version, and a two-partner version, to cater to your assistance situation. The technique uses the Contract--Relax technique to improve the final position (in a range of movement sense) and adds adjustments and micro-movements.
The large-scale effects on the body are 1) the shoulders sit further back on the rib cage, so more under the ears; 2) the thoracic spine is straightened dynamically, and under load, and 3) standing and sitting posture is improved.
Consider this the complementary stretch to the hip flexors: these two exercises are the two major ways of changing your spinal alignment for the better – assuming you are like most of us (slightly kyphotic, and with an anterior pelvis tilt).
For athletes, this (together with the hip flexor stretch) help you to run on your legs, instead of in front of them, and will help cyclists breathe more efficiently when on the bike.
Find the audio file of this video at https://supplementary-material.s3.us-west-2.amazonaws.com/Partner+pec.+minor+stick+stretch.mp3.