I love this sequence from 5.20 - it looks very much like your basic boyfriend trying to impress/scare girlfriend for fun scenario (men are strange as are women).
And he grins. ‘See, honey, I’m fine. Cool huh?’
Joan gives Bethesda the stink eye for being part of Sherlock’s game and then lets him have it. Apparently JW feels free to beat on Sherlock if warranted (see season 3 punch to the arm and several verbal threats). Perhaps it’s the best way to get thru to him that she is upset …
And he is confused as to why she’s mad and yells back
Anger is followed by explanations …
* * *
The below is a bonus - I found this almost amused look on JLMs face while screencapping Joan “attacking” him