not spiritual thing. it's an internal power practice thing, if you know the details. same goes for funakogi undo. you can go through the motions which most folks do or work with some of the internal experts, and the information they will tell you on those things would make your head explodes.
Phi, Phi, Phi,
It's a dessert topping AND a floor wax!
Seriously, this is not an "either-or" or even a "both-and." The entire misogi-no-gyo
is a complete mikkyo/sanmitsu practice in its own right, involving the training of the mind, body, and speech. These practices can be found in multiple lines of transmission, some more complete and some less complete. The "internal experts,"(or at least one of them, though I'm not in the endorsement business) have a wealth of key information on the body aspects of the practice that I have never encountered anywhere else. That said, I should also note that I have encountered orthodox Buddhist and folk shinto ritual specialists who have information on the speech and mind aspects of the practice that is comparatively rare and no less critical if one is approaching the set of exercises that make up the misogi-no-gyo as a complete practice with soteriological goals beyond the merely physical.
Each line of transmission seems to have emphasized some elements and de-emphasized others, in keeping with the interests and proclivities of the people in that line. While the internal power aspects really are quite amazing and valuable, in the long run, I would argue that those too are just flashy tricks in comparison to the deeper purposes of the training. (Of course, the long run may include many kalpas of cyclic existence.
Or maybe it just feels that way during the third set of reps of the last exercise in whatever routine you happen to be working, if you consulted with one of the IP guyz and followed his advice.)
Rev. Barrish of Tsubaki Grand Shrine of America has extensive experience with these practices in a fairly straightforward Shinto context; he sometimes posts here, and this might be a useful occasion for him to do so.