I will give you an example: the book “Light on Yoga,” by BKS Lyengar is commonly considered a modern classic on poses. But take a look at this… for Salabhasana (lobster pose, shown in the photo), the book says this about the benefits of doing this pose:
“Since the spine is being stretched back, it becomes more elastic, and the pose will alleviate pain in the lumbar spine and sacrum. In my experience, the people that suffer from herniated discs have benefited from the regular practice of this asana, without being forced to rest or get surgical treatment. The bladder and the prostate also benefit from the exercise helping to keep them healthy.”
Actually, in this position, the muscles of the spine (to be more exact) do not stretch back, they contract. Which does not necessarily mean that they become more flexible. In the case of herniated discs, even though it is true that it may relieve pain for many, for others it can exacerbate it. If you have a nice therapeutic yoga that helps you, you can continue to do it without causing pain.
Getting back to the purpose of this conversation: there is not a single test done that says that the bladder and the prostate do in fact benefit from this pose.
You have to ask yourself, where do have they gotten their information? Partly, it is the personal experience of what they feel in their own bodies. Another part is that which they have been taught by their professors or even some books. And perhaps another part could be what the professor heard from when someone else was teaching and chose to adopt it into their teachings.
But, how do we know if those conclusions are correct? We don’t. 
Because without real scientific research, we cannot assume that what worked well for Pedro is going to work at all for Jose. Even if it seems to have helped 20 people working with one professor, there are many variables that could have occurred while arriving at the results. It’s kind of like saying something like this: “when in this pose youreceive what feels like an abdominal massage, the organs in this area can become stimulated which will heal this area.” I know, it sounds circumspect, but I think that this is all that I can responsibly say. It’s that o many are not objective declarations and proven tests, and thus can be an all out lie. And don’t we want the truth
During teacher training, when they taught methodology of teaching, they made sure everyone understood this point. As Yoga instructors, our work today is to separate that which we know from what we believe. This distinction must be conveyed to our students… our effort in maintaining our obligation to the truth… Right?
Mijael.
