Why didn't Hayate aim for Baki's head here?

He could have just killed him on the spot since Baki didn't even flinch during this technique, but Hayate instead aimed for the most armored part of the body and got his sword stuck in the armour...

Was he holding back? can you do this technique only if you aim for the shoulder, or was he actually trying to split his head open, but missed because of his sleep deprivation and covid?