There has always been this huge debate in Martial Arts and Combat Sports, with regards to the position of your hand i.e. fist. This is my theory; I believe there is no right answer. I have come to the conclusion that it depends on the application. But what I will say is when teaching a new striker whether for just self defense or combat sports that ‘Thumbs up “ is my suggested option. Which is also similar to Bruce Lee’s JKD (Jeet Kune do) and other self-defense striking styles. A lot of times attempting to turn your thumb down too early in the strikers progression starts bad habits such as elbow flaring which in turns telegraphs your punch and you begin to just arm punch. I am a huge fan of trying to keep elbows close, this will help with maintaining the kinetic chain from the floor. Arm punches have their place such as blinding, or hand fighting which I will get into later. But I would prefer to have the habit of delivering body weight strikes in the beginning instead.
The main take away is keeping the thumb up prevents reaching with your strikes, which exposes you offensively and defensively due to lack of balance and the dreadful elbow popping when you miss. If you are reaching, your head is usually over or worst case past your lead foot. Offensively, you become compromised because without balance you cannot effectively throw combinations / multiple strikes especially from the floor (bodyweight) to include lower body as well. Defensively, you are exposed
to the uppercut or the check hook. So as you progress to a thumb down position, be conscious of head position and balance. I think the thumb up, in self-defense protects your hand better when hitting a hard surface especially if you don’t have gloves also. I have yet to prove this but that is my theory, bottom three vs. top two knuckles. The wrist appears to stay more stable, which is the challenge in striking. That is why hand wraps are so important. Pad and protect the knuckles as well as support the wrist. New strikers usually easily hurt their wrists from hard punches if they are not careful.