I wanted to check if it is possible for me to jump through with straight legs. What I see is: my anatomy allows me to jump through. I only need a bit more flexibility. According to Gregor Maehle it should be possible to jump through with straight legs if one is able to lift the feet up from the floor while on flat hands.
Uddyjana bandha helps to go deeper into this pose. The shoulders always move away from the ears.
I engage my legs nowaday when I bend forward, not only because I need strong legs for some advanced asanas, but also to protect myself from injuries.