Exactly. When firing live ammunition, the bullet seals the barrel which results in gases behind it building up high pressure. In the case of a straight-blowback action (usually seen on compact, small caliber pistols [.380 ACP or less] and certain submachine guns), those high pressure gases, which are pushing rearward with the same force that they are pushing the bullet forward, push the action rearward to cycle it (the empty case acts as a piston in this case).
In the case of a gas-operated action (like the M60 that Stallone is firing in that picture), there is a hole in the barrel near the muzzle which leads into a gas tube which leads to a piston which is connected to the bolt. Some of the high-pressure gases are diverted into the gas tube which forces the piston and the bolt that's connected to it, rearward, cycling the action.
Without high-pressure gases neither system will work. Putting a restriction in the barrel to increase gas pressure duplicates the effect of a barrel-sealing bullet being in front of the gases, which solves the problem.
In the case of recoil-operated actions (typical of service pistols, 9mm Parabellum and higher), in addition to needing a restriction in the barrel, they also need to be converted to straight-blowback by using a fixed barrel (as opposed to the typical tilting barrel) with no locking lugs (because the blank cartridge doesn't have anywhere near enough recoil to cycle the action). The same thing applies to small caliber conversion kits. For example, if you buy a .22 LR conversion kit for a 1911-type pistol, it not only converts the caliber, but it converts it from a recoil-operated action to a straight-blowback action as well, and for the same reason that it needs to be done with a blank-firing adapter.
The trend in modern movies and TV shows is to use CGI to make it look like the action is cycling, like empty cases are being ejected, and like muzzle flash is coming out of the barrel, so all they need for that is off-the-shelf, unmodified firearms. I don't like that trend because it doesn't look real. Firing blanks inherently looks real because it is real (aside from the lack of recoil with the types of guns that normally have substantial recoil, but that applies even more to CGI, because they actually aren't firing anything at all from the guns).
On the other hand, it does allow for more options that would be unsafe with blanks. Blanks aren't 100% safe; those gases coming out of the muzzle can cause injury if the muzzle is close enough to someone. Plus they are loud, which can cause hearing damage if precautions aren't taken. For example:
While filming Terminator 2: Judgment Day, Hamilton suffered permanent hearing damage in one ear when she fired a gun inside an elevator without using her ear plugs.
The John Wick movies are good examples of the expanded options you get with CGI gunfire as opposed to using blanks.
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