A good back strengthening position in advanced yoga is the half frog pose. Some other good positions in my opinion would have to be the upward facing dog, the locust pose, and the king pigeon pose.
a really good way to stretch out your back is called a swan stretch. you lay on your stomach and arch your back up with your arms supporting you this will let your back release or tighten wich...
Yoga has a lot of schools with varied postures and positions. There are a lot of videos and and instructions to help you understand it and practice better.