kapala = skull
bhati = light (implying perception, knowledge)
Step by Step
Kapalabhati consists of alternating short, explosive exhales and slightly longer, passive inhales. Exhales are generated by powerful contractions of the lower belly (between the pubis and navel), which push air out of the lungs. Inhales are responses to the release of this contraction, which sucks air back into the lungs.
Focus on your lower belly. Many beginners aren't able to isolate and contract this area. If needed, cup one hand lightly in the other and press them gently against your lower belly.
Now quickly contract (or pump your fisted hands against) your lower belly, pushing a burst of air out of your lungs. Then quickly release the contraction (or your hands), so the belly "rebounds" to suck air into your lungs. Pace yourself slowly at first. Repeat eight to 10 times at about one exhale-inhale cycle every second or two.
As you become more adept at contracting/releasing your lower belly, you can increase your pace to about two exhale-inhale cycles every second. Imagine the exhale sweeping out or "brightening" the inner lining of your skull.
Do 25 to 30 cycles at first. Gradually increase the number of cycles you do each practice to 100 or more.