I wrote a paper on Samurai nearly 40 years ago.
Seppuku was modified multiple times throughout history. Originally the samurai would open their own belly and then cut their own throat if possible. This was later modified to add the second to do the beheading and then eventually instead of opening their belly they would merely place the tip of their sword or even a fan upon the belly before being beheaded.
Most don’t know that the women of samurai families had their own ritual which was basically throat cutting vs belly cutting.
Reminds me of the movie “Harakiri” in which a masterless Samurai is forced to kill himself with his own sword, the blades made of bamboo.
Another change involved when the Kaishakunin (the second) was to deliver the daki-kubi (coup de grâce).
Initially (back when being Samurai really counted for something) the Kaishakunin was to wait to strike until the condemned had completed his cuts (the number and location of which also changed over the centuries) and returned the tantō to the tray it had been offered on. In the latter Samurai era the ritual was watered down so much that all the condemned need do was to stretch out his hand as if reaching for the tantō and the Kaishakunin would strike.
Being Kaishakunin was both a great honor and a great responsibility because the condemned's obligation (to his lord, etc) was considered fulfilled once he had completed the ritual cuts. The Kaishakunin's role was to end his suffering once this ritual duty was dispatched.
And it took a swordsman of considerable skill because his duty was not to decapitate the condemned but only to sever the spinal cord but leave the head still somewhat connected to the neck. Depending on the source, this was done because it would have been in bad taste for the condemned's severed head to roll out amongst the guests present, or because if it fell off and rolled around, the head might become soiled. Since the condemned was restored to an honorable status at the moment of death, it would have been a dishonor to allow his remains to become soiled.
Done to best effect, the head would fall into the lap of the condemned, who would be sitting cross-legged in the lotus position. That way it would appear he was holding his own head.