In Ashtanga second series there are a lot of deep back-bending postures, which can be very pleasant and beneficial for some and maybe too intense for others. Back-bends not only integrate movement in all sections of the spine, but also integrates your limbs while doing the posture. By using the whole body you can avoid over-using your most flexible parts of your body (which could be: shoulders, spine and hips).

Why SHOULD we be practicing backbends

The most obvious reason is that it opens up and stretches out your chest and shoulders. Most people have a tendency to hunch forward as big part of our days we spend by the computer or in our phones. So by doing any variation of a backbend, depending on your level, will greatly improve your posture.
Backbends, especially where you start lying on your stomach, strengthens your back muscles and the spine as you need to use all the back strength to lift the body up from the ground.
Backbends like Wheel pose (Urdhva Dhanurasana) or Camel pose variations (Ustrasana) are very good for shoulder and hip-flexor stretching. Also most of the time during the day we are bending forward (sitting, picking up something, walking), and by practicing Ustrasana or Urdhva Dhanurasana we open up the whole front of our bodies.
Also as stated above backbends stretch out the whole front of the body, which improves your lung capacity. Always leaning forward also minimises the lung capacity and the breath becomes short and shallow. By opening up your shoulders and chest you release the tension and give more space for the lungs to expand and your breath becomes deeper and there is a larger capacity of oxygen inflow in your body.

What to think of when practicing any kind of back-bend.

Some people, when doing back-bends, find it better to squeeze their buttocks and some find it better to keep it relaxed. Some, when flexing, might feel compression in the lower back or at the sacroiliac join and vice versa. It really depends on the personas. We all are so different. So try and see which version is better for you.
Lower back is the most bendy part in our spines. That keeping in mind, try to intensionally bend more in the upper back. This can take the pressure off of your lower back and will include all parts of spine in action.

Pasasana Benefits

In Ashtanga yoga, Pasasana is the first seated posture in second series. Pasasana is a deep twisting and balancing posture. For this posture it really helps if you have longer limbs and more flexible ankles/ achilles. For those who cannot do the full version of the asana, there are a few modifications and adjustments that can help you get the same benefits as from the full version.

For example I cannot lower my heels to the floor in a full squat so I need a yoga BLOCK under my heels to feel stable and to be able to twist and wrap the arms around the torso and legs.
If you cannot clasp your hands behind your back you can use a STRAP by holding on to it.
If your heels are not that far from the ground you can ask your teacher to help you lower them down by gently holding the top shoulder with one hand and pressing down the same side knee with the other hand without getting you off the balance.

Noose pose is a great twist and stretch for the spine. Furthermore it reduces tension and stretches out the shoulders and upper back. Due to the squatting position the ankles and knees are strengthened.

Benefits

Can relieve asthma
Relieves back, shoulder and neck pain
Strengthens ankles and knees
Improves digestion by squeezing and massaging the internal organs
Relieves menstruation discomfort

Why should we practice back-bending

Pasasana --> Noose Pose

Pasasana Benefits