Making a traditional fist, clench tightly, and then try to move the hand to the left and the right using the other hand to grip the hand making the fist. It's hard, but it can be done, especially moving the hand backwards towards the forearm. Now repeat with the thumb on top, pressing down with the thumb on the top knuckle. Try to rotate the fist with the other hand again. It should be much more difficult to move, and not that much 'thumb pressure' is required to create that 'lock'.
I have experimented quite a bit with fist formation, given my age and relatively poor conditioning of my knuckles and so on. By repeatedly hitting the bag at various speeds and with various amounts of power, my goals have been to have a more relaxed punch (until the very moment of impact), keep my wrist straight to avoid hurting it, and to avoid hurting my knuckles or the budding arthritis in my finger joints.
One thing I am absolutely certain of is that the Isshinryu punch we practice is vital for keep my wrist stabilized and having a relaxed punch until the moment of impact. I cannot claim this is true for every human being, but it has always been the transmitted wisdom among Isshinryu karateka and I have also found it to be so. Many think the unique thing about the Isshinryu fist is that it is vertical and delivered without a turning motion, and this is true, but to me what really matters is that thumb on top. Especially the light pressure that serves to lock the wrist (it also helps to turn the fist ever-so-slightly downwards, so that the top two knuckles hit first, which is our way).
The funny thing is, I've heard criticism of the IR fist for being 'vertical' all the time, which in practice it is not. We don't torque it, true, but we turn it to fit the circumstances. I would not deliver a vertical fist as an uppercut for example. But the fist formation remains the same in all cases.