There are many risks with breath play, some immediate and easily recognized -- others can go largely unrecognized in the short term but have real consequences. At least a working knowledge of what is going on with the human body during breath control as well as the health condition of the sub is essential at well.

The following is a helpful resource:
http://www.text-solution.com/breath.html

I'm sure I am preaching to the (experienced) choir here, but this is a type of play that should be taken very seriously. Be safe and have fun!