For this technique one has close the eyes and put all his attention on the spinal column.

Keep the backbone straight and erect at the same time relaxed. Visualize it and see it in your minds eye.

Become aware of two spots first the region of the tailbone the sacrum and middle of the back of he head.

Now join these two spots with awareness and begin to run awarerness up and down the spine.

Also begin to add the flow of breath to it.

When the breath flows in let the attention move upwards from the tailbone to the centre of the back of the head. 

And when you breathe out ket the atention travel downwards from the centre of the back of the head towards the tail bone.

Then consolidate your awareness on this pathway with the breath.

It is through our spinal column that our life flows that is why in this sutra it is used as tool of centering ourselves.