Yoga sutra 1.28
taj-japaḥ tad-artha-bhāvanam
The systematic repetition of AUM (ॐ) should be accompanied with a profound meditation on its meaning
You should not repeat AUM chant mechanically, but chant with full awareness. Visualise the vibrations moving up the spinal column from start to finish of the AUM chant. After all the air has been expelled from your lungs and the chant ceases, you should then pause a little before drawing in another smooth, long breath in to chant the next AUM.
Damian Cadman-Jones is an authorised teacher of Prana Vashya Yoga™