|"Reach With Your Prayers" (Photo by Najmat)|
In class I explain this as a sharp exhale caused by pulling the lower belly back to the spine (I like to engage mulabhanda or the "root lock"). The contraction is felt behind the lower ribs or the lungs. I've had one teacher use the image of a bellows, a quick compression that forces the air from the lungs, so it can refill with new breath.
The Yoga Journal describes this practice in detail along with other yoga techniques for pranayama.
Go here also for more exploration of pranayama techniques:
And keep asking questions!