An activation is one of the ancient and medicinal keys within any yoga pose that creates wellbeing.  This fourth key got lost in the transition from East to West and this is why very few yoga teachers in the West include them in their pose instructions.

They're unique to each and every pose.

If you don't apply ALL four keys within your pose, you'll miss out in creating a, abundance of lasting energy within the body.

Yoga is not a hobby or something you do... it's created within the body - as a FEELING.