In taiji alchemy it's pretty tough to understand the language and terminology they use. It just sounds like mystical mumbo jumbo, and complicated networks of energy channels, sublimation of energy.
Taoist alchemy uses hundreds of pages to describe how to attain optimal physical health by purifying and sublimating the different networks of energy, whereas Ajahn Lee just concisely expresses it something like "the four elements of your body will become harmonized, and you'll feel very comfortable." (big understatement)
Here's my attempt to fill in the blanks between Ajahn Lee, EBT, and real taiji.
Your body has earth property in there, which I'll call "ice".
It has water property, which I'll call "water", or "juice".
It has heat property which can range from cold to hot, I'll call "fire".
It has wind property, which I'll call "the force."
It has space property, which I'll allude to with "water dissolving into vapor, and emptiness".
So if someone who's earnest about getting good samadhi, jhana, here's the process. Say they sit four hours a day. Even two hours a day you should feel some gradual improvement day to day.
So when you're sitting, you just have to do two things.
1. Gently try to make your body more relaxed than it is.
2. Gently try to make your mind calmer, more relaxed, and peaceful than it is.
You'll know its working because you'll feel a force that pervades everywhere in your whole body. everyone's health condition is different, so maybe at first you'll only feel the force pushing in one spot, a piece of ice somewhere, maybe the size of a small stone. It may feel like that chunk of ice is permanent and will never melt. But if you sit every day, it will gradually melt, you'll feel the ice shrink, and melt into water.
So the chunks of ice in your body will gradually be targeted by the force, and slowly melt. The process may be physically painful. When I first started meditating, there was this chunk of ice in between my lower and middle back that would only appear when I sat down to meditate, and not have any pain outside of meditation. It took about 3 years for that small chunk of ice to fully melt and become pain free.
Probably the reason most people don't get jhana is they don't believe that the meditation will ever be pain free, and give up.
Pain is just the interaction of fire melting the ice in your body into water.
So the more you meditate, the more you become horrified (for most people) when you come to the realization that most of your body is ice. Because its mostly ice, that is what "normal" feels to you. But if you persist in your meditation every day, and you relax properly, that ice surely will melt. And when enough ice melts, and enough of the main energy channels in your body are free of blocked ice, you'll feel the water (that is the melted ice) now has a clear path,
and then BAM!!!
You get a taste of the juice. The juice (piti sukha) is the water flowing freely and continuously around energy channel loops. The juice triggers pleasure chemicals in your brain.
When enough of the ice in your body is melted into water, and the main channels are unblocked, you'll be able to get first and second jhana. It will feel like liquid pleasure coarsing through your veins. Second jhana water movement can be so intense its like you're sitting under niagra falls. Not everyone will experience it that intensely, depending on their age, health, etc.
Then the fire continues to melt ice and vaporize the water into emptiness. So from a stiff block of ice of a body, it will feel like a smooth, comfortable bag of water, and then become lighter and lighter. when the water melts into vapor and dissolves into emptiness and visual light.
Day by day, it improves. I don't think anyone who meditates according to the EBT (consistent with how Ajahn Lee teaches it) would be surprised if they levitate one day.
So here's an important point that the Hindus and taoists who teach real taiji will explain the "why and how" in detail, but the EBT does not. This is probably because monastics following vinaya, and following the Buddha's schedule (AN 3.16) are already doing the right fundamentals, but this leaves Buddhist laypeople out of the loop on a critical detail.
Celibacy is essential. True brahmacariya, meaning you try to not even have one microsecond of lustful thinking, or any negative emotion at all.
I need to wrap up this post, I'll just say IMO, without celibacy even if you meditate four hours a day you put your ceiling at a mediocre first jhana and low quality second jhana, and you may never see much visual light.
And even much worse news, if you have jhana ability, then being non-celibate has greater risk of developing psychological problems and nervous system disorders than someone who is non celibate and can't do jhana, especially the older you get.
In Chinese, the jing-shen is the common phrase used to express health. Jing means the exact same thing as viriya in the hindu sense. It's energy, vigor, and reproductive potency, semen. In chinese, the same character in "health" (jing) also means "sperm". "shen" corresponds with the abundant visual light in the 4 iddhipada.
The further away one goes from celibacy, you'll not have steady and stable (if any) visual light (shen), and your lack of jing is directly proportional to your health.
Unfortunately modern science and psychology has very inverted understanding of celibacy and sexual health from a spiritual aspirant.
An earnest and even modestly skilled meditator can test this out themself. Follow the buddha's schedule on basic walking and sitting meditation, and sleeping schedule (AN 3.16)
If you're maintaining celibacy well, you'll directly feel how strong viriya is. The four hours a sleep, plus one or two naps a day, will feel like plenty. Even less than four a of day sleep if you're meditating a lot. Visual light will be become abundant, your mind will become sharper, memory will improve, your mind will be quick and agile, as your body softens and improves, it will be free of disease, light, comforably, reflexes get sharper.
If you interrupt celibacy, you will drastically feel the impact it has following the schedule above.
taiji = supreme ultimate = 4th jhana
I'll share my notes on all of this (back problem prevention, back repair, cross leg sitting, yoga, taiji, etc.) in a lot more detail one of these days, it's going to be a few hundred pages at least. It's impossible to impart the important points in a short post in a way most people can understand. I'll probably have to shoot some video and pictures, but even still, I don't think anything can replace in person demonstration and check up by someone who knows how to fully relax.
Real taiji quan is based on taiji. And real taiji doesn't start until first jhana, and doesn't really take off until fourth jhana. Taiji quan without real taiji is just slow motion walking and arm flapping. Real taiji leads to 5 of the 6 psychic powers. Same 6 as the EBT abhiñña, minus the 6th one with the destruction of the asavas. That's why taiji is "supreme ultimate."
Ajahn Lee's "keeping the breath in mind" is also the underlying basis for the way into real taiji. I re-read his booklet frequently. There's lots of brilliant instruction in there, but some of it is expressed so concisely one won't realize the brilliance of it until they've gotten a taste of it.
I think the difference between 4th jhana and iddhi-pada in the EBT and real taiji differs in that the Taiji practitioner would apply his knowledge and skill to ensure a healthy physical body, whereas a buddhist who frequently practices jhana probably would get most of the physical health benefit without trying, but because they're not paying attention to and learning from the process (how the 4 elements in the body harmonize and become smooth, empty) and/or don't care about physical health, they can have a few health problems that start off as minor and eventually become significant in their old age.
But many frameworks can get the same end results as real taiji, jhana, and iddhi pada. Even among christian meditative traditions, Essenes, Hindus, you can find people who have jhana, divine eye, levitation, everything short of destruction of the asavas. They have different labels for various states of concentration and so forth, but I think you'll find in common among all of them the same basic ingredients as in the iddhi pada: samadhi, bliss, abundant light, etc.