Intensity is a mental attitude more than a physical attitude. Many people misunderstand what intensity means. They think it means straining and sweating. No! That is a wrong meaning of the word! Intensity is to get totally involved, fully immersed and absorbed in what one is doing. Intense practice means a fast and keen mode in adjusting, correcting, and progressively proceeding.
B.K.S. Iyengar (2009). “B.K.S. Iyengar Yoga Wisdom and Practice”, p.112, Dorling Kindersley Ltd