Sustainable means that you want to be able to continue doing yoga without injuries. More often than not, people get injured doing Yoga because they forgot to maintain a good posture foundation.
Some people do more importance to breathing than how a posture (exercise) is performed. In reality; both are equally important; nonetheless, a good foundation will prevent injuries and if we want to maintain our practice through time, this is the way.
Tim Senesi has a good foundation series in his website but you may access for free his beginner guide series at YouTube.
To maintain good practice is important to use Props . Blocks are commonly employed for various reasons and in practicality it could be implement in any pose. For example, Utthita Parsvakonasana (extended side angle pose). The most important is keep a correct alignment, the block will elevate your body to get enough space for the chest rotation and flexion without compromising the spine.
In the above picture Tim demonstrated how to perform the pose with the correct activation of the hamstrings, quadriceps and gluteus medius. In this pose the weight of the torso falls over the front leg, so you need the proper strength and length. Up until you manage all these muscle activations and have the proper length the block will give you a better chance to protect your spine, gain strength and to visualize. The action in all these variation are more eccentric than concentric due to the relation of gravity and arm weight. In this pose (extended side angle), abdominal breathing will provides asymmetrical stimulation to the diaphragm helping you to developed more strength and stamina.
Blocks comes in many materials but I prefer solid wood. Solid wooden have better weight and can be used for other propose. The majority of wooden blocks are hollow and light like bamboo, cork or composite (foam materials).
I will recommend not only take a class or watch an instructional video, but also to read a good anatomy book specializes in yoga.
Check your local stores and yoga center for more information and reject those teachers that doesn’t like that their student use props.