You load the M-1 with an 8 round clip. You push the clip in with your thumb, the bolt closes, and you get your thumb out of the way while the bolt closes- hopefully.
When you are not paying attention, you forget to remove your thumb in time and the bolt slams into your thumb. It happens to everyone ONCE. It does really hurt.
I’ve never had it happen while loading a clip...only when closing the bolt on an empty chamber—when you HAVE to put your thumb into the “line of fire”.
(And if it only happens ONCE, then there’s two people out there who’ve gotten away with not getting it, at MY expense!)
You can also “cheat”, and hold the bolt handle back with your right hand while you insert the clip with your left. The clip clicks in, you let go of the handle, and your thumb is not in the way.