I guess when Haiji is able to put more force into his strikes, he can start to do damage to places he could not have before, which is why he suddenly starts seeing weak points.
I wish Haiji's powerup was cooler: it is basically Haiji but faster instead of changing his approach to combat. Also, it is supposed to be harmful, but in what way and why?
At least this chapter shows that the manga will attempt to balance its doctor side with battles by having Haiji attempt to heal his enemies instead of harming them. I'm not sure about the execution in this chapter, but hopefully future battles will do this better.