This is a fascinating video of B.K.S. Iyengar teaching yoga in 1968, in Bombay. Look at the mix of younger and older students (there's at least one child, and at least one rather old man). Look at how they're doing Sirsasana (headstand) in the middle of the room. And look at how the neck is extended in Sirsasana in the man shown at 2:30. He's not going to hurt himself in headstand. (I'm of course thinking of that recent NYT article saying how people will hurt themselves in scary poses such as headstand and shoulderstand -- not if they're taught well!)
Even back in the late 1960's, Iyengar and his students show such wonderful extension through the arms, legs, and trunk, keeping their chests open and well-lifted.
There's a vibrancy to the poses as taught in the Iyengar Yoga method (and as shown in this video) that I don't see in many of the other "styles" of yoga that are commonly taught today.
I can tell when "experienced" yoga students come to Harmony Yoga for the first time, whether or not their experience is in Iyengar Yoga. If their yoga background is from another method, there's rarely the extension and vibrancy that we see in poses from experienced Iyengar Yoga students.
(P.S. Iyengar gets his "right" and "left" instructions confused in this video, while he's "mirroring" Virabhadrasana II -- as many of us yoga instructors also do from time to time!)