Roller-delayed blowback is fairly simple once you can see how things work together. The bolt has some fore and aft movement on the bolt carrier, and when the bolt locks into battery, a wedge that is separate from the bolt forces two rollers into indexing notches in the barrel extension or trunnion. After the cartridge is fired, the bolt and carrier are forced backwards under pressure. HK weapons flute the chamber around the cartridge to allow some gases to assist this action and ejection. this is what causes the lines on spent brass from HK weapons. As the bolt components separate, the angled portion of the wedge allows the rollers to move into the bolt and unlock from the trunnion or barrel extension. This action delays the unlocking long enough to be a viable and clever operating system, although sometimes violent with larger calibers. I hope that was clear enough.