Bandha Yoga on the psoas muscle. The article offers a sequence of poses that first contract and then release the psoas, which runs roughly from the lumbar spine, across the back of pelvis and attaches to the front of the femur.
Psoas is a hip flexor, so it either raises the knee or (if the leg is fixed) bends the torso down. Consciously focusing on psoas during standing poses will activate it and give you the power of this important inner muscle.
Their sequence uses modifications of standing poses to get the hamstrings out of the way so you can really use the psoas. Check out the sequence and see for yourself. Interestingly, the authors say that having awakened psoas, unrelated poses like arm balance somehow become lighter and freer.
The sequence ends with stretching the psoas in purvottanasana (which I found much more accessible after this sequence) and supported setu bandha.