When I was first learning  Iyengar yoga, the classes I attended were two hours long. I found the intensity of them overwhelming at times and would take myself off to the toilet a couple of times during the session, just to have a break.
The teacher, Martyn Jackson, had a booming voice and would exhort his students to do more, even though sometimes we were at our limits. Poses were held for long timings and repeated. As a beginner, I remember my legs shuddering and shaking in standing poses, so now I can commiserate with new students in my classes whose bodies are just learning to wake up.
It’s so interesting, the process a person goes through when strong sensations arise. One might run away from what is happening, which is sort of what I did when I took my toilet breaks. Another possible reaction might be to try to change the situation – perhaps make a case that one shouldn’t have to go through discomfort in yoga. Still another approach might be capitulation, just limping along and hoping to get through as best you can. And a final and more skilful method could be accepting what’s happening and staying with the process moment by moment, observing sensations. In this way of being, there’s the space to do something or not while avoiding knee-jerk reactions.
B.K.S. Iyengar says that the reason for his intense way of teaching is that he is trying to awaken students to be present, at least for the duration of his class. He describes what happens:

As I shout at them to straighten their legs…they cannot be wondering what is for dinner or whether they will be promoted or demoted at work. For those who habitually flee the present, one hour’s experience of ‘now’ can be daunting, even exhausting, and I wonder if the fatigue felt by some students after lessons is due more to that than to the work of performing asanas. – Light on the Yoga Sutras of Patanjali

While I have a lot of difficulty finding release in a pose while being shouted at, I can easily find stillness every morning I practice in the peace of the Yoga Shed. And I find that holding the poses gives time to touch what’s happening in my body, and by extension my heart and mind.

Ksanatatkramayoh samyamadvivekajam jnanam

Focusing with perfect discipline on the succession of moments in time yields insight born of discrimination.*
*The Yoga-Sutra of Patanjali, translation and commentary by Chip Hartranft.