A skillful yoga practice can be considered a tool, a tool that can inform the mind, reform the body and ultimately transform the self.
The magic lies not in the tool itself (a hammer alone cannot build a house) but in the skill of the practitioner using the tool.
A master craftsman doesn't need a hundred tools.
She needs to know WHAT tools she has access to.
She needs to know WHEN and when not to use them.
And through practice and learning she needs to learn HOW to maximize the effectiveness of the tools she has available to her.