Hmm. From the excerpts I read from the links, it sounds disastrously New Age. But most of it seems centered around forms of meditation (they call if "attunement"😉, which can at least be good for personal well-being.
Also:
A natural healing system and spiritual system, Reiki is very simple to practice. With clear intent and strong pathways, the practitioner becomes the vessel through which Reiki is focused and distributed. Described as feeling warm, soothing and "tingly", a recipient of Reiki will feel measurable heat emanating from the practitioner's hands. This is normal and most often invited.
...I was expermineting the other day with thinking about various body parts. I could get them each to tingle slightly if I focused on them long enough (amusingly enough, my balls were the most responsive), a natural consequence of focus. Try thinking about your own tongue, for example, for about 3 minutes, and tell me it doesn't start feeling weird. Also, the body produces heat. I'd be surprised if I didn't feel heat from someone's hands close to me.
In other words, I'm guessing a lot of Reiki isn't anything beyond stuff like that, though I'll admit I've never encountered it until now so it's hard to say for sure.