Observing the breath vs Pranayama Is it better to just watch the breath as it unfolds or sometimes we need to control it to slow it down ,calming down and noticing it in the same time ?.
Calm down the mind - breath is indicator of mind state. 

https://plumvillage.org/sutra/discourse-on-the-full-awareness-of-breathing/


  
  ‘Breathing in a long breath, I know I am breathing in a long breath. Breathing out a long breath, I know I am breathing out a long breath.
  ‘Breathing in a short breath, I know I am breathing in a short breath. Breathing out a short breath, I know I am breathing out a short breath.
  ‘Breathing in, I am aware of my whole body. Breathing out, I am aware of my whole body.’ He or she practices like this.
  ‘Breathing in, I calm my whole body. Breathing out, I calm my whole body.’ He or she practices like this.
  ‘Breathing in, I feel joyful. Breathing out, I feel joyful.’ He or she practices like this.
  ‘Breathing in, I feel happy. Breathing out, I feel happy.’ He or she practices like this.
  ‘Breathing in, I am aware of my mental formations. Breathing out, I am aware of my mental formations.’ He or she practices like this.
  ‘Breathing in, I calm my mental formations. Breathing out, I calm my mental formations.’ He or she practices like this.
  ‘Breathing in, I am aware of my mind. Breathing out, I am aware of my mind.’ He or she practices like this.
  ‘Breathing in, I make my mind happy. Breathing out, I make my mind happy.’ He or she practices like this.
  ‘Breathing in, I concentrate my mind. Breathing out, I concentrate my mind.’ He or she practices like this.
  ‘Breathing in, I liberate my mind. Breathing out, I liberate my mind.’ He or she practices like this.