Trawling for jhānic sludge while orbiting the spine advancing through the first two jhānas

 What Taoists call 'The Microcosmic Orbit'

Some Taoist schools can make things overly complicated, so I don't recommend doing research on their teachings on the topic 'The Microcosmic Orbit' unless you know what you're doing and have good reason to that research.

My main objection is they make it seem like you have to do one of their special breathing techniques to open up the orbit. 

The reason I refer to this concept, is because it's well known, well documented, and universal to all meditation traditions whether they have a label for it or not. We all eat, breathe, sleep, etc., and if we keep 8 precepts, relax deeply in all 4 postures and don't dissipate our vital energy through too much talking and thinking, then our jhāna battery is going charge up with internal energy, and jhānic force and heat is going to increase in power. The breathing just has to be relaxed, one doesn't have to follow weird and complicated breathing techniques. 

Where do the 4 jhānas and  'The Microcosmic Orbit' overlap?

Meditation, and the 4 jhānas are not an exact science, so these are general ideas and can vary with various health problems meditators have. 

But in general:

* The microcosmic orbit is open and operating in any healthy living being. If it wasn't, you'd be gravely ill and die. So it's not like the orbit was closed and then it's only being opened by meditation experts. 

* what is referred to as 'opening up the orbit', refers to a noticeable increase in the jhānic force pushing through blockages along that orbit, in the direction indicated, starting from the tail bone. 

* When there's jhāna constipation, that orbit will feel blocked in various places. 

* When it opens up, then you get the sukha (physical sensations of pleasure corresponding to endorphines) of the first two jhānas, and the characteristic hydraulic feeling, of strong forceful watery jets of current in the body, especially the microcosmic orbit. This is what second jhāna simile refers to with the natural spring at the lake bottom that's feeding the lake. The tailbone is where that spring is, and the direction of the water flow, if you were sitting in the lake on that spring, would like like the jet of water forcefully travelling up your spine. 

* Third jhāna and higher, your body is saturated with water, you don't feel a flow. It's like a bottle half empty being shaken, there are strong forces of water current. But if you shake a full bottle, you don't feel current of water. Also, if you shake an empty bottle you don't feel force of water current. 

An ordinary person who indulges in sensual pleasures is like the empty bottle.

A competent Third jhāna meditator with a charged battery, is like the bottle full of water. But if you do energy draining activities, like giving dhamma talks, thinking deeply about suttas that drain your jhāna battery, then the next time you meditate for some number of minutes, you'll temporarily feel the forceful hydraulic second jhāna sensation as your jhāna battery charges back up. 

Trawling for jhānic sludge while orbiting the spine

This is an interesting effect that may be noticeable for some people at some times during the process of your microcosmic orbit is opening up.

It happens when a big blockage is finally melting down (after months, years), and as the blockage dissolves over days, weeks, it feels like it breaks into sludge and the sludge is slowly being trawled around the microcosmic orbit revolving around the spine. Eventually the sludge completely dissolves, but in the meantime every so often you feel the sludge slowly being trawled along the orbit, by the jhānic forces charging your jhāna battery.

The interesting thing is, this isn't necessarily a one time thing that you go through passing through second jhāna the first time you attained it while working on third jhāna. It can happen whenever a pretty big blockage has a break through. 

As your jhāna battery is being charged and getting rebuilt into a bigger and stronger battery over your years and decades of practice, it's like your microcosmic orbit keeps expanding and adding new lanes to the highway, and then three dimensionally more lanes getting stacked up on each other, like an onion growing more layers of magnetic energetic gooey stuff.  And when a good sized energetic blockage is breaking down, you may feel that old sensation of 'trawling for jhānic sludge' again, except it will be on a different layer of the onion as your previous sludge meltdowns. 

Conclusion

This is one of the interesting effects on your jhāna journey you may or may not notice. It's proportional to how big and serious the blockages are when they melt, and how subtle your perceptions of bodily tactile sensations are. They're like comets and other transient phenomena, if you notice them, they can be quite interesting and add to your personal understanding of jhānic forces and jhānic heat melting down blockages, resulting in a body that feels lighter, stronger, more pliable.