Part of the ‘magic’ of karate was the adept could strike out when unexpected. The fact they were trained was not obvious, allowing the opponent to face an unexpected response to their actions.
I was working on my tai chi palm training, which I often do these days. To more retain ability than anything else. So while I was doing so, my eyes observed the back of my hands as I exercised.
That brought many training sessions over many years to mind. Ones which worked on many open back hand strikes.
As single strikes they can slash down against the bridge of the nose, down against the orbit of the eyes where the back hand knuckles fit the face. Strikes by Sherman Harrill descending into the neck where the hand fits into the neck below the jaw, strikes to te temple, striking into the armpit, rising strikes into the groin, strikes into the solar plexus, descending strikes into the points on the upper chest, rising strikes into the under side of the jaw. Of course this is not a complete listing of possibilities.
Then there are the compound striking possibilities,
I am limited these days having no one to work with to make photos showing what I mean.
That being the case I turned to Bing pictures. I found almost nothing, which surprised me.
I could only find one photo which surprised me, and at that not exactly what I wanted either.
With all the internet karate technique discussion, now that I think about it, I don’t recall it having been a topic of discussion. Of course I do not know everything, perhaps just I haven’t heard of it being discussed. It may be a primary strike of your style. If so please excuse my lack of knowledge.
But for me I very much serves the purpose of something that an opponent will not expect.
I hope my students will recall when I used this, Others, may this serve as inspiration.