ELI5 Cupping Therapy - can bruises really heal you?

Why do we see many professional athletes look like they were attacked by a giant octopus? How did a treatment from ancient history become the biggest trend in modern locker rooms? If a bruise is "stuck" blood, how can it actually help your blood flow? Can we really suck "bad energy" or "toxins" out of our muscles with a glass jar? Why do some doctors call this "quackery" while others say it's a powerful mental tool?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: breeze_in_the_trees, tateshinski, phage0070, doingthehumptydance, mohamed and zhang

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