As i point out in another post of mine, many people here are confusing "Hatha Yoga" with "Raja Yoga". The problem with BKS Iyengar/Krishnamachar/Pattabi Jois etc. schools is that they have mixed up both and sold it to the West and thus the resulting confusion.

Patanjali in his "Yoga Sutras"(main text for Raja Yoga) only says "posture should be steady and comfortable" (note asana = posture). Thats it. He also says nothing detailed about Pranayama. It is the internal mental aspects that are crucial in his system. Whether you do it sitting in a Chair, lying on a Bed is immaterial.

On the other hand, in Hatha Yoga the physical aspects are all important and are defined by Asana, Pranayama, Bandhas and Mudras. Here is where the forceful exercises come in. By a combination of body and breath, you try to induce certain sensations in your mind which is often defined as "Laya" i.e. "dissolution".