What is Kriya Yoga?

The up going currents through sushumna are cool currents. These are SOMA.  Down going currents are warm currents. This is Agni. So while doing Kriya we are giving Ahutis of Cool currents i.e., Soma into Agni. The sound Haaa is the Beejamantra of Shiva and the sound eeee is the Beejamantra of Shakti. Sushumna will be getting purified  with these sounds i.e.,haa and eee while doing Kriya. With the purification of Sushumna the darkness in the chakras will be removed. That is the driving away of ignorance of man due to indulgence in sense pleasure. Then you will hear the highest Mantra i.e., OM.  This is called veda vidhi.  Veda means hearing. Vidhi means duty. So preparing for hearing of OM is your Veda  vidhi This is what is described in Chapter4 29^th verse in Gita. Appane Juhwati praanam, praanepaanam tathaapare, praanaapaana gateerudhwaa praanaayaana paraayanaaha. To offer as sacrifice the incoming breath of Prana in the outgoing breath of apana and vice versa is real Yagna.