Setu bandhasana ... Always a good time! I was totally baffled for a long time as to why this was the prep for backbends in primary series. In my ignorance I thought something more like Dhanurasana or a nice stretchy bridge pose would inform my body how to backbend much better than this pose which I lovingly referred to as straight-jacket-asana.
Flash forward 7 years later and I now understand backbends have much more to do with strengthening than stretching. Rooting through the legs and orienting the femur bones in the pelvis to aid in finding the low belly and protect the sacrum is most of the work. When the lower body is solid, the upper body has more freedom to expand. Think of a clamp on a garden hose...the lower body is like the clamp and the upper body like the water pressure building. This action creates space, making the backbend safe.
Setu Bandhasana demands this activation by literally putting your neck on the line. So of course this no-nonsense, elemental backbend appears in primary series, which among other things is a primer for the rest if the ashtanga practice. My teachers Jen Rene and Peg Mulqueen keep the basics fascinating by reminding me that the work put in primary will serve me down the road when the poses start to get fancy pants. This is because it's all the same story on the inside, even when the pose changes externally. Another beautiful example how the practice is symbolic for what the yogi seeks, oneness in a world that seems so different on the surface.