Anatomical How-to Guide
There are two obvious ways to reach your ass: from the front, as I've always done, and from the rear, as the lady does in the video. I bend better forward, the North Col route, so to speak. By rounding my torso in the direction of my ass, voila! my arm reaches closer.
Putting my fingers in involves bending my hand with the wrist down and the fingers up. I've found things to sit on, and that pushes up through the bent wrist towards the destination. For example, squatting down, pushing my wrist onto a big plug or dildo that is resting on the shower floor.
For bending forward, the sternum has to move downward, in the direction of its length. But that's not enough. In the back of the spine, the erector muscles have to lengthen. Longer in the back allows shorter in the front. The ribs in the back separate, and the ribs in front fold together like the ribs of a Japanese paper fan.
The South Col approach involves turning to face the side. The shoulder and arm are behind you, the shoulder blade pulled backward near the spine. Now, if the ribs on that side can fold together, magically the arm and hand will creep towards the ass, towards the tip of the tail bone. For the ribs on one side to move closer the ribs on the other side have to open. The ribs are what enables the spine to bend where we want it, and the ribs protect the spine from bending where we don't want it to bend. Most people don't allow as much movement in their ribs (and therefore spine) as is anatomically possible for them. Our self-image gets constrained.
The path of the sternum in this variation is much more complicated because the top of the sternum is pointing behind to help the shoulder around, and the bottom has to poke out forward for the back to arch. The arching of the spine with folding of the ribs brings the hand down towards the pelvis. She made this look simple to do. No extreme stretches, just a nice accommodation throughout involving her ribs, shoulder blades, spine and pelvis. The arm is just a passenger!
Since it's not easy for me to fully fist, I sometimes do an approximation. I put in a moderate-size toy and then reach my fingers in and around it. The toy simulates the fist, and your fingers and wrist move it like you would a fist.
Have fun and don't stretch too much. Visualize your goal. Include all parts of your body in your image.