At its core, all qigong essentially boils down to coordinating your breathing and movement, with focused attention; yi dao, qi dao, li dao (attention arrives, energy arrives, power arrives).
Shaolin is famous for two sets of qigong in particular; the yijinjing and xisuijing (although you'll be hard-pressed to find any two teachers who agree on exactly what they should look like).
The yijinjing, loosely translated means "muscle/tendon changing classic". This is the set that Master Jiru teaches as part of his approach to "mindfulness of feeling".
Xisuijing means something like "marrow washing exercises". Dr. Yang Jwingming writes "Xi means "to wash" or "to clean." Sui includes Gu Sui , which means "bone marrow," and Nao Sui , which refers to the brain—including cerebrum, cerebellum, and medulla oblongata. Jing () means "classic or bible." This work is commonly translated "Marrow Washing Classic," but "Brain/Marrow Washing Classic" is a more accurate translation."
In I Liq Chuan, GM Sam Chin teaches that expand and condense helps to cycle the qi from the center of the bones (the marrow) out through the ligaments, tendons, and skin and back and considers this training to be xisuijing.
Other systems of kung fu, like Little Nine Heavens, teach specific exercises using weights tied to the genitals as a major component of, if not the singular focus of xisuijing.
<---WARNING: long, but relevant tangent ahead--->
In his book "Qigong, The Secret of Youth" Yang, Jwingming translated many old documents detailing the practice of xisuijing, and while there were exercises that involved the testicles, no mention was made of swinging weights from them, therefore I'm inclined to think that either only certain schools adopted this practice as part of their specific approach to marrow washing, or over time the practice was abandoned by most schools.
Remember the semi-mythical founder of Zen, Bodhidharma, was an Aryan prince turned monk who'd traveled to Shaolin from India around the 5th century AD, where he found the monks in poor health due to lack of exercise (they spent all their time translating sutras from Sanskrit into Chinese).
As the story goes, Bodhidharma taught the monks various yogic practices, which became the foundation of Shaolin's qigong and kung fu. There are Indian yogis who also practice hanging weights from their genitals, so it almost certainly pre-dates its practice in China, and potentially was an important part of the original xisuigong.
In his article on the purpose of xisuijing, Yang Jwingming writes
"Most important of all, the practitioner of Brain/Marrow Washing Qigong is able to lead Qi to his brain to nourish it, and to raise up his spirit. To the Daoists and Buddhists, Brain/Marrow Washing Qigong is the path to reach the final goal of enlightenment or Buddhahood."
How does one lead the qi? With focused attention: yi dao, qi dao, li dao. Where the attention goes, energy goes.
Now that we've established some background on marrow washing and the role of focused attention and breathing in qigong methods, let's take a look at some recent discoveries that support the premise that we can use intentional breathing exercises to optimize the health and function of our brains.
Our brains are our most metabolically active organs; they account for only about 2% of our body weight yet consume ~20-25% of all the calories we eat; as a result, it produces large amounts of metabolic waste products. Accumulation of these harmful substances, like amyloid plaques, are associated with cognitive decline as seen in Alzheimer's, and one of the most important functions of sleep is clearing out waste from the brain by circulating (i.e. "washing") the CSF, or cerebrospinal fluid. Breathing however also plays a role in the process!
A recent study published in the journal Nature Scientific Reports showed that slow breathing had a considerable impact on the flow of CSF.
The volume of CSF circulated was four times greater from breathing than from heart rate, and this was for a "normal" breath rate of 15 breaths per minute.
On Instagram, Dr. Steven Lin writes "During an inhale and exhale the chest rises and falls. The change in pressure flows upward to the CSF dynamics around the brain.
Here’s how it works:
Breath in (inspiration) – Lowers chest pressure and empties the venous plexus. CSF flows down the spine.
Breath out (expiration) – Increases chest pressure and fills the venous plexus, pushing CSF up the spine into the head."
Slower breathing, like that used in qigong and breathwork, can be 5-6 breaths per minute (or less).
In an article from Science Norway, study author Vegard Vinje explained why fewer deep breaths have a greater impact on the flow of brain fluid than faster, shallow breathing. Essentially, the longer waves that result from deep breaths can carry more volume. He compares it to ocean waves hitting the land.
“Imagine a beach with rubbish. A long wave will remove garbage and clutter on a beach more efficiently than a short one,” he said.
Although there is a certain risk of trying to shoehorn modern data to fit into ancient practices, I believe that the old masters developed deep insights into the inner workings of their bodies and minds, and modern imaging technology is finally allowing us to see and measure what they were feeling all along.