One way to test and refine the power of ki and to keep it from being an intellectual game:

First thing in the mornings, Japanese yamabushi often took a cold water bath, the misogi, naked or wearing only a thin white robe under a freezing mountain waterfall. You can do this in your shower. Turn on the water freezing cold. Standing outside the stream, steady your breathing, Focus on the Seika Tanden in the Hara (the “one point” about two inches below the navel and in inch or so inside.) Visualize it as glowing and then as spreading warmth through your body, up through the crown of your head. Extend your Ki higher with each breath. Step into the stream of water and crouch under it, keeping your head high and your back absolutely straight. If you are breathing and using your mind correctly there will be no shock, your breathing will remain calm and steady, and the flowing water will seem to merge with your skin — the cold will feel blissful and purifying, not painful.

You may have to practice this many times. The first time, you will almost surely lose your breath as if it had been knocked out of you with a mallet made of ice. Stay under the water, keeping your muscles relaxed, and shout (or just exhale, forcefully), “Hah!” Do it as many times as you need, focusing all your energy on the sound, until you no longer feel shock and agony but only the blazing bliss of sheer awareness.

-The Book of Zen & Haragei http://diamondsutrazen.blogspot.com/p/book-of-zen-haragei.html