Like Spiritual Energy those who posses can survive attacks that would normally kill people and can be used to extended one's lifespan to hundreds or thousands of years though this appears greater extent as older members such as Sojiro is noted to over 10,000 of years old.
Similar Magic one's emotions can help strengthen attacks power, Reiki can do the same making them stronger.
Reiki is formed into an Art that is almost completely unique to watcher using it though similar ones exist. These Arts are special abilities and range to Fire manipulation to Telekinesis etc. However while it appears can used by all watcher's not all of them can form into an Art.
However Reiki can also enhance one's physical abilities making faster, stronger or even given improved senses.
It has noted the Reiki can't be absorbed by techniques that are power by other energy sources other techniques will not work or just reject it. This has noted Yakuma that it is the most superior energy sources in the universe.