The M1 Garand rifle has a strong, heavy action. The user must be careful when releasing the operating rod to close the open chamber when the rifle is unloaded. The Garand uses en bloc clips, which are pushed down from the top of the receiver into a built-in magazine. Once you have pushed the en bloc clip all the way down into the magazine, you'll feel it go *click* twice. The first time is when the magazine locks in, and the second click (which you really have to push down to get) is the click of the operating rod releasing. If you don't have a magazine in the rifle then the only way to release the rod is to press the magazine follower all the way down to the bottom of the magazine. You use your thumb so that you can keep your fingers outside the action, held just in front of the operating rod handle - that way, when you press down far enough and the rod releases, the handle slides forward and your bladed fingers catch it before it can slam closed. That lets you pull your thumb out of the action before releasing the rod fully.
If you do this wrong, the action closes while your thumb is still in the magazine well. Usually, it will be lifting out, and the very tip of your thumb or thumbnail will get caught by the bolt as it moves into battery and get SLAMMED into the chamber.
This hurts quite a bit, and leaves a lovely dark bruise on your thumbtip or under your fingernail. In extreme cases, it may nip off a few millimeters of your thumb tip.
This condition is known as 'M1 thumb.'