"You must fill every inch of your body with the asana from your chest and arms and legs to the tips of your fingers and toes so that the asana radiates from the core of your body and fills the entire diameter and circumference of your limbs. You must feel your intelligence, your awareness, and your consciousness in every inch of your body." B.K.S. Iyengar
A practice where you can focus on creating a feeling of extension:
More on extension from B.K.S. Iyengar in his book, Light on Life:
"One should not overstretch, nor understretch. If one thing is overstretched, something else gets understretched. If overstretching comes from a swollen ego, then under stretching results from a lack of confidence. If overstretching is exhibitionism, understretching is escapism. Overstretching and under stretching are both wrong: always stretch from the source, the core, and the foundation of each asana. This is the art of dynamic extension. It is not yoga that injures, but is the way one does yoga that leads to injury. The moment space becomes narrow, it means you are injuring. In the correct asana, there is no narrowness. Even if your body is stiff, you have to bring space".