Iyengar yoga is characterized by precision and alignment, timing in and sequencing of postures, and the use of props.
Precision and Alignment
The precision in an asana is not merely meant for the alignment of the part but for the proper functioning of the individual. If the body is aligned with precision, the breath is aligned with the same precision; if the breath is balanced, the mind, emotions and senses are balanced.
Iyengar Yoga practioners are known for their ability to stay for a prolonged duration of time in different poses. For a beginner, this means developing will-power which is essential in early stages of practice. However, one has to evolve beyond the realms of merely external force or will. Not staying for staying's sake, but performing an asana where one intrinsically wants to stay in the pose. As our practice evolves, we can stay longer, but we find the effort required to maintain it lessens.
As Iyengar Yoga practitioners, we are aware that is not just the asanas but how you perform them, how long you perform them, and also the sequence in which they are performed which determine their effect. The sequence is determined by purpose of practice, weather, time of day, health of the practitioner, and level of practice. With over 200 asanas, there can be any number of permutations and combinations. However, there are certain rules of thumb. For example, Salamba Sarvangasana (shoulder stand) is never practiced before Salamba Sirsasana (head balance). Also, practice generally ends with Savasana (corpse pose) or other relaxation asanas.
Props have been a fruit of Guruji's innovative genius. It is because of the prop that people of all ages and health status can perform asanas with ease and attain the benefits of the practice of yoga.
Hierarchy in practice
One final aspect of Iyengar yoga is the concept of hierarchy in practice. While a beginner might be taught Utthitha Trikonasana (Triangle Pose) in their first class, Guruji practices Trikonasana after 70 years. While the beginner's Trikonasana is performed mainly on the muscular-skeletal level and is guided by his/her teacher, Guruji's Trikonasana is performed in a state of meditation and guided by his citi (consciousness).
(Excerpted from an article by Prashant Iyengar for the publication Yoga Rahasya, a YOG publication)