How to do? Tie off a limb, then lift with light weights. E.g., tie surgical tubing around the upper arm and then do bicep curls. Why? It's typically used with debilitated people (e.g.: elderly, burn patients), who can thereby get benefit similar to using normal/heavier weight. Dangerous? You bet. The creator of it even once gave himself a blood clot which became a Pulmonary Embolism. Any surprising mysteries? Yes, e.g. tying off the upper arm gives benefit in the chest muscle. Kaatsu is probably the last thing that people here would want to actually do. But musing about it might provide some insights into CFS and PEM. It's not known why Kaatsu works. Maybe it's from building up waste products as if the exercise was heavier. Maybe it's from hypoxia, or maybe from blood pooling.