M1 Clips don't "work" they hold the rounds and the weapon does the work.
Yes, the en-bloc clips DO some work. The answer is so simple you can't see it - The M1 en bloc clips are springs in and of themselves. If you insert three or four rounds, they roll around uselessly. But when you insert eight rounds, you use force to insert the last ones and this force is stored in the walls of the clip, which are strained. The inserted cartridges keep the walls tensioned, and thus the spring "loaded." In a strict application of classical physics, the en-bloc clip is doing work on the cartridges. It is doing so because of stress on the molecules in its internal structure - thus it is working as a spring.
In the strictest sense, the M1 clip is an integral and removable part of the magazine. Most magazines can function without their accessory loading clips (like and SKS or an AR, or a 1911). The M1 is one of the few weapons where the clip is actually PART of the magazine. Especially since the magazine is useless without the clip providing both the feed lips and lateral cartridge tension.
None of this disputes the above comments about clips loading magazines, etc. The bulk of an M1's magazine is fixed inside the rifle. But I point it out to highlight the actual mechanical detail and effort, as well as manufacturing technology, that went into this design. Modern rifle users tend to think of the M1 system as antiquated; it wasn't - it was actually very advanced and ingenious. Box magazines already existed - it wasn't that they didn't think of them. The Garand system was designed so that ONE, high quality, tuned to the gun magazine would be retained within the gun, and cheap and simple tensioned clips inserted into it to provide it's lateral tension and feed lips. Spare ammunition could be carried with much less bulk and weight (an infantryman's nightmare), logistics were simplified, and for a prone infantryman the reloading process was quicker and safer. All in all, it was a heck of an ingenious system.