It’s a feeling, you can’t actually do it. In fact it’s the act of not-doing… definitely a verbal contradiction.
It’s a feeling, and, like all feelings, is only possible to explain by comparison or with a simile (try explaining what an orange tastes like) whilst hoping that the person, to whom you’re attempting to explain the feeling, has had experiences that are similar to your own. In other words, it’s nigh on impossible.
What does it feel like?
Sediment; it’s like the sediment of a murky pond settling on to the bottom.
It’s like ‘playing dead’ when you were a kid, with someone trying to lift you up.
It’s an object falling to the floor, the moment it fully impacts the ground.
It’s feeling gravity and borrowing it.
It’s the letting go of every cell in your body.
It’s feeling your own weight.
It’s no longer holding on.
It’s the sensation you can get in the second before you fall asleep.
It’s how you can feel when you meditate.
It’s how your nervous system feels when truly calm.
More esoterically, it’s a feeling of connection to the planet, or to the earth.
What’s its function? It connects movement in tai chi and qigong (well actually any movement in anything), but is a movement in itself; it’s the out-breath before the in-breath, as well as the in-breath before the out-breath. If it’s true that any object or event is possible by virtue of the fact that its opposite exists, then it’s one side of the coin. In other words, its function is to allow its opposite to exist.
I’m aware that this is beginning to sound a bit ‘zen’, (‘What is the sound of one hand clapping?’). So to bring it back to the practical, it’s what you experience when you are jumping off the floor into the air. In the second before jumping, just as you finish dropping your body, you sink your qi; it’s the connection between the dropping and the rising – a softening.
If you try to sink your qi, you fail. You can watch this happening if you do the jumping exercise above. When you attempt to control it, you stop softening and start directing the muscles. All spontaneity is lost.
This is a bit like (back to the sediment again), if you try to make the sediment sink in the pond, you just end up stirring it up.