This post was deleted for the following reason:
The art isn't great and the writing doesn't make up for it ()Artist's commentary
Stratagem
The whole thing about miko having to show armpits jokes are overly done, but I wonder if there is any possible explanation, as in, if exposing armpits may provide advantage in a danmaku battle.
One hypothesis is like the comic above, that somehow, everyone in Gensokyo may have a fetish towards armpits and be distracted by it, not dissimilar to fighting a woman of ample bosom, sporting a major cleavage.
Or another hypothesis of mine is, Reimu's body odour may interfere youkai subconscious, similar to how some experiment suggest that male odour may have subliminal effects towards female. Reimu's showing her armpits simply allowing the odour to spread, and then combine with the possibility that youkai may have a heightened sense of smell compared to human's....
These are all are not to be taken seriously.
...
I'm so gonna get misunderstood by people not getting the context...