The bandhas are subtle internal contractions that direct the flow of energy in the body while at the same time protecting the alignment of the spine. For me, the bandhas have a brightening and vitalizing effect on my practice. When I am upside down, they also serve to increase my proprioceptive awareness. Engaging the bandhas in handstand, for example, helps me to feel connected to my center, or plugged in, so to speak. Physically and spiritually.
In yogic thinking, the sushumna nadi is an energy channel that is believed to run through the center of our spinal cord. We can think of the sushumna nadi like a garden hose. If the hose is pinched at any point along its length, the water cannot flow freely. The primary function of the bandhas is to provide the internal strength necessary to hold open the sushumna nadi and to direct the flow of prana within it.
Pincha Mayurasana is more accessible to me now than it used to be but it still puts unhealthy pressure on my mid back. It still pinches the garden hose. I can feel that the energy is blocked and I have a hard time connecting to - or controlling movement from - my center of gravity in this shape. So for right now pincha is simply a physical posture that hasn't come to life yet for me at the subtle, energetic level. But I am excited to start playing around with it more. I look forward to continuing to open my shoulders, to deepening my awareness of what is happening at my core, and to being able to take the kink out of the garden hose so that the current can flow through.