If we want to get really technical, the proper way to form a fist is to:
1. Stick your thumb upwards and splay all your fingers straight out.
2. Fold only the first two joints of your fingers until the tips touch their bases.
3. Fold the final joint, thumb still sticking up.
4. Fold in the thumb without rotating its base. Rest the tip of the thumb on the inside curl of the index finger, with the tip itself firmly planted on the second joint of the index finger.
If you form a fist by putting your thumb inside it, the force of punching might sprain or even break the thumb. If you form a fist with the thumb outside your clenched fist rested, if the fist glances its target or gets pitched up at the wrist the thumb might end up receiving the brunt of the force of the blow and also get sprained or broken. If you form your fist correctly as described above your thumb should become immovable on impact. Try it.