This is a really lovely and helpful Meditative Breath Exercise .. a wonderful place to start your meditation, especially if you find your mind is agitated or excessively active.
Also, if you find you are having one of 'those' day then try a few rounds of this 9 Round Breath Meditative Exercise to settle your mind and stabilize your focus as a precursor to a more sustained meditation practice.
In this exercise you are imagining that you can alternate your breathing through only one nostril at a time.
The instruction is to use just your mental focus to imagine (as strongly as you can) your breath flowing into and out of the chosen nostril. Note that we are just imagining!
Firstly... settle into your seat... take a few moments to soften and relax your body... find a position of comfort... and let go...
1. For three rounds of breathing - imagine as strongly as you can that you breath in through the left nostril and out through the right.
2. For the next three rounds of breathing - reverse the process : breathing in through the right nostril and out through the left.
3. For the final three rounds of breathing - focus on breathing in and out through both nostrils.
And then .. return to your normal rhythm of breath and observe how the breath has rounded and softened a little.
Before you finish... see if you can give your complete attention to each full breath cycle - feeling the full movement of your breath... from the desire to take the in-breath; to the need to release and let the breath go.