I think it (Yoga-Shakti) awakens quite naturally the moment one takes the resolution to do the yoga. If the resolution is sincere and one has an aspiration, it wakes up by itself.
In fact, it is perhaps its awakening which gives the aspiration to do yoga.
It is possible that it is a result of the Grace… or after some conversation or reading, something that has suddenly given you the idea and aspiration to know what yoga is and to practise it.
Sometimes just a simple conversation with someone is enough or a passage one reads from a book; well, it awakens this Yoga- Shakti and it is this which makes you do your yoga.
One is not aware of it at first—except that something has changed in our life, a new decision is taken, a turning.
Ref: Questions and Answers 1955