Read David Swenson’s reply:
I appreciate your acute exploration of my texts in regard to the details of vinyasa. It’s difficult to convey in words the energetic dynamics of bandhas. In what you’re describing, there’s an emphasis on lifting the tailbone and weighting the hands. After we jump forward and up, becoming airborne, and the weight has shifted to the hands, we must tilt the pelvis and tuck the tailbone under as we start to move down and through the arms. This is a necessary phase of the jump-through. We cannot jump through with an absolutely straight back. Maybe that is what you are experiencing when you say that you end up “rounding” your lower back. That could be a necessary component of that phase of the jump-through. This curling action can correspond with a contraction of the abdominal muscles, but this action should not be confused with bandhas.
Misunderstanding about the bandhas abounds. They should not be confused with abdominal strength or contraction of the erectus abdominus muscles. The engagement of bandhas initiates deep from our pelvic floor and is not simply a contraction of the abdominal wall. When you say that this rounding of the lower back is occurring when you engage the bandhas, it makes me think that you are perceiving the bandha contraction as an abdominal action, rather than one initiating from the deeper core of the body.
Also keep in mind that the true purpose of bandhas is the direction and regulation of energy flow through the subtle channels of nadis. Just because someone can jump through does not mean that they understand or correctly use their bandhas. The bandhas are subtle tools, and they transcend simple physical mechanical action. The longer we practice, the more these subtle tools come into play.
The final bit of advice I have is to enjoy your practice regardless of the ability to jump through. We tend to place too much emphasis on the external aspects of the practice. These abilities are fleeting at best, and the “real” yoga is what we cannot see.