Board Thread:Off Topic/@comment-31105409-20170429025502/@comment-25356303-20170501132213

As stated on the article:

When power attacking with the blade, it sends out a horizontal or vertical energy blast that inflicts 30 points of damage, depending on the type of power attack used.

I haven't played Dragonborn in quite some time, but if I recall correctly the energy beam is red, not rainbow.