If extension is from the intelligence of the brain, expansion is from the intelligence of the heart. While doing asana, both the intellectual intelligence and the emotional intelligence have to meet and work together. Extension is attention, and expansion is awareness (…) When one stops straining and the brain is passive, it becomes spiritual yoga. When you have extended to the extreme, live in that asana, and experience the joy of freedom in that asana. While stretching, you must always create space and extend from your center. Compression is bondage, and expansion is freedom.
— BKS Iyengar, Light on Life