Props

In many Asanas we may use props such as a wooden block, a blanket, a belt, a chair, a bolster and even the wall and the floor will indicate us in the right direction of from where the action and movement should come from.
For example, in Swastikasana, translated to English as the Easy Crossed Legs Pose, we use the wall (see photo below) not just to support our back, but also to help us understand what alignment actually means. By using the wall as a reference to a correct alignment, we can evenly sit on our setting bones, and use them as our base, as our root, we can actually start comprehend and learn how to straighten our back, lengthen the back of our neck, opening our chests and thus our hearts. If we would not have had the wall as our guideline we might have collapsed our shoulders, closing our chest and believing our spine is elongated by mistake and by habit. Since habits play a major role in our posture, we tend to believe that whatever feels comfortable is "normal" and therefore cannot do much harm. However, what we interpret as comfortable is merely a habit, which can indeed be harmful. But luckily we have the wall and many other props to teach us alignment and a correct healthy posture.

"The wall is my Guru"
B.K.S. Iyengar