This was one of my all time favorite parts of the whole movie. 100% serious right now
And considering his injury, it actually makes sense for him to do this. At this point, he doesn’t have the same adrenaline levels as later on (adrenaline overrides your pain receptors to a point) and he needs to generate more force for his swings. By using his right arm (uninjured side), as well as any abdominal muscles on that side, he gets that force. He then does the twirl to gain momentum (more force for the down swing) and THEN finally brings up his left arm for the final extra bit of force he can put into the actual blow. The entire move also has the advantage of getting Finn a little off balance due to dodging the swing.
My only contribution is that it’s been found that if mosquitoes suddenly all died off, there would only be minor consequences until the niche was filled by existing insects. I forget what the article I read was called, but we learned that in both Insect Ecology and Forest Health.