Correct on the API, but to clarify a bit. The early primer ignition actually fires the primer and begins powder ignition BEFORE the bolt fully closes, so the backward movement of the case must fight not just the weight of the bolt and spring, but the forward momentum of the bolt as well. The result is to make the bolt act as if it is a lot heavier than it is. That in turn allows the bolt to be relatively light and the operating spring to be light enough for easy manual cocking.
When API is not present, that is when firing from a closed bolt (as required by BATFE rules), the gun must either have a heavier bolt or have springs much heavier than used in the open bolt models. For example, the standard Thompson M1928 SMG can be easily cocked with one finger, where the Kahr Arms semi-auto requires a hefty effort to do so.
Of course, blowback operation of any kind is only feasible with low pressure cartridges; with higher pressure cartridges or less breechblock weight, a breech lock of some kind is necessary. The same .45 cartridge that can function in a blowback SMG with a heavy bolt normally requires a locked breech in a pistol unless the slide is made very heavy (Hi-Point). Blowback operation is generally not feasible at all with rifle power cartridges. I once read that a .30-'06 blowback would require a 30 pound bolt, a bit heavy for your favorite deer rifle.
Jim