Kihon: How to punch to the rear from Heiko dachi, parallel stance #2 

Asai karate; 2023 Mumbai seminar
浅井空手・基本：平行立ちで真後ろに突く実演：2023年, インド講習
In Asai ryu karate, flexible upper body is required. The kihon exercise you see today is very unique and can be found only in Asai karate.
In this video:
I stress that the elbow of a punching arm must not be bent. There is another method of punching, that is called Kagi zuki 鉤突き. You bend your elbow on purpose and generate power differently.
The motions are similar to the techniques you find in Heian sandan. There, your stance is Kiba dachi 騎馬立ち and the punching arm is bent as you punch.
Thus, Kihon technique shown today is totally different from that of Heian sandan. On the other hand, I find several techniques in Tekki shodan that require this upper body rotation ability.
Unfortunately, most of the practitioners do not (or cannot) rotate enough in those techniques.
Once you get used to doing this with closed fist, try Teisho uchi 底掌打ち and see if you can do this quickly and powerfully.