Here is a shorter, less wonky version of the post below.
If I can get to that stable place Patanjali is urging us toward, and keep my tremor-prone nervous system and mind resting below the level of conscious thoughts --and importantly, thoughts with words in them-- then the breath can flow relatively smoothly. If I am thinking, and again, especially word-thoughts, those citta vrttis go straight to my throat and yield a tremor. Minimal mind activity = minimal tremor.
This is why pranayama does not work for me much at all in any sort of public setting like a pranayama class. I can't go down far enough in my nervous system to get underneath the tremor impulses because the voice of the instructor pulls me up to the place where I am thinking, comparing, remembering, evaluating, etc., and this quite naturally creates vrttis in my mind.
Ironically, the voice of the instructor is supposed to create vrttis in the student's mind, just not unhelpful ones. We go to class in the first place in order to understand ourselves or the world differently, to learn new and better ways to think and act. It's a bit of a Catch-22, no?
I think in fact that this is everybody's problem. Everyone has trouble getting their vrttis to be quiet, in class or not; if we didn't, we'd all be liberated already, without having to practice our whole lives (or for a multitude of lives if you believe in reincarnation). Only, the average person can outwardly look like they are doing it right. I have no such luck. I never could hide what I was thinking.