I was taught in my jujitsu school to make a fist with the thumb on top which I discovered through some internet digging is the way Isshin ryu practitioners make a fist. I have yet to find anyone else that does this. I am wondering where this comes from. I believe I have heard it called a Chinese fist. I assume that this is where it originates since karate's roots are Chinese styles such as White Crane. Can anyone give me specifics as to the history of this method of fist-making?
I don't know how authoritative it is, but this article credits Isshin ryu's founder as deciding on the thumb position through experimentation:
For over 30 years, Shimabuku experimented with different methods of holding the fist. He found that if you placed the thumb on top of the fist, instead of on the side, the wrist became stronger. He therefore settled on this position for the Isshin-ryu vertical fist.
As an Isshin-ryū practitioner I know sensei Shimabuku had trained Japanese officers during WW2, so the infusion of a large amount of Jujitsu in Isshin-ryū wouldn't be alarming to say the least. Not to mention he was very fond of Kumite and Kobudo, so if the vertical fist was stronger/faster for him, I would not doubt one bit he built upon what worked well.