After watching that clip, I have one more theory. Since it was Irene that calls Holmes away from the fight, it is possible that he is reminding himself to not let his feelings/emotions for Irene get the best of him and allow him to make a mistake.
He says this before plotting out his attack pattern on his target to clear his mind and focus on taking this guy out before going after Irene.
Assuming I'm wrong, another reason could be a reminder Holmes gives himself before planning a fight out to not feel bad for the amount of pain he is about to inflict upon his opponent(s).
(This explanation is wrong, but I'm leaving it in anyway as why not?)
It's possible he wasn't breaking the fourth-wall, but rather speaking to his opponent who wouldn't let him go.
The man was about to attack Holmes, and Holmes essentially warns him to not let his pride get the best of him "Mustn't register on an emotional level" referring to the man's pride of losing the fight.