Back before I could get the jhānas, when I had jhāna constipation,
I attended a long retreat where one of the teachers was a nun with a background in Zen,
as well has having spent several years in Pa Auk Forest Monastery.
From her background, it was clear she was skilled in VRJ (Vism. Re-definition of Jhāna), and certainly able to do EBT jhāna.
During the retreat, aside from good general and specific meditation tips, she did try something interesting to clear up my jhāna constipation. She did not explain it to me or warn me ahead of time. It's only years later after I cured the constipation on my own that I could reflect back on the incident and figure out what she was doing.
I had stayed an extra week after the official retreat ended, and meditated diligently. One evening while I was doing a long night time sit, suddenly there was a loud noise from the storage room next to the room I was meditating. It sounded like a large book or object was slammed onto the concrete ground, louder than if it had simple dropped accidentally from the force of gravity.
If you're in deep meditation, or even attempting deep meditation, a large noise can cause a shock. Internally, I felt a jolt of energy, but my jhāna constipation remained.
There were only 2 or 3 people in that area at that time, so I knew it was the nun who 'accidentally' dropped the book making the loud noise in the next room. The shock of the noise evidently is enough to cure some jhāna meditators of constipation.
2 or 3 years later when I was finally able to do jhāna, the reason was the energetic blockage I had in my chest for so many years, had melted enough for energy to pass through to the belly. It felt almost exactly like a drop of water dripping from my head area, slowly dripping down the neck, down the chest, finally settling into the belly. Along with that drip of water, was a mini orgasmic euphoric physical/sensation.
Over the years, the thread of water passageway got bigger and bigger. Imagine there's a river that flows from high to low. The high point is at your tailbone, the head is a waterfall with a dam, and the belly is the low point. jhāna constipation is like having a dam. In my case, the dam finally got some cracks in it, and water started trickling through. After that dam broke, there was still sludge that would move very very slowly along with the flow of water.
Now instead of the river being a line, now imagine it's a perpetual flowing loop. This loop starts at your tailbone, flows toward you middle back, then top of your head, then down your face, chest, into the belly, back to the tailbone.
So after my dam in the head area broke, and it would feel like sludge would slow revolve in the body in that loop. It would take several minutes for the sludge to make one revolution. Gradually, as the energetic blockages melted more and more, then the sludge would revolve faster and faster, until finally all the sludge melted, and on a single inhale the breath energy ascends my back to my head, and a single exale it circles back into the belly.
Everybody's health condition is different, so there are all kinds of weird and different ways the blockages melting can subjectively feel. But typically, people will have big blockages, constipation areas in the low back, middle back, neck, head, chest, heart. When it's constipated, it feels heavy there, blocked, with force and heat trying to push past it. Just like you need to fart and can't fart, need to burp but you can't burp, need to excrete but you can't. It's very uncomfortable. And if nerves are impacted, it can be quite painful too.
One clue you have jhāna constipation, is you have certain body pains only when you meditate, but the pain magically disappears the moment you stop meditating.
This is because any time you attempt to meditate correctly, via 👑😶 noble silence 🚫(V&V💭) a-vitakkaṃ a-vicāraṃ samādhi, force and heat are pushed through all the subtle energy channels in the body in an attempt to permeat every cell of the body.
So if you have sufficient passaddhi/pacification/relaxation, when you flip the 👑😶 switch, jhana constipation discomfort and/or pain comes on, when you cease 👑😶, then pain/discomfort immediately stops.
The reason zen masters sometimes will hit a meditator with an incense board, or shock them with a loud noise, is sometimes the meditator doesn't have enough passaddhi/pacification, holding on to some physical or mental tension habitually, perhaps in sympathetic reaction to their meditation pain/discomfort. The shock of being hit by physical force or a loud noise is enough to momentarily relax the blocked area enough for piti+sukha to surge through.