"We looked at various ways to couple and decouple the valve," Rosario explains, "and then we looked at the lifter and the oil pump. We now have a lifter within a lifter with an engine oil gallery. So when oil pressure acts against this locking pin, it dislodges it. That allows one section to collapse into the other section, and when we take the pressure off it locks again. With the absence of oil pressure, it's in the V8 mode. With any loss of oil pressure it would stay in V8 mode; that's the default mode."
The system shuts off every other cylinder in the firing order and brings them back online in 12 to 20 milliseconds, which Rosario says is imperceptible. "A hard launch already has a 250 millisecond response lag," he points out, so less than a tenth of that won't be noticed, and it's done in "a seamless fashion."
These are opinions that test drives in two full-size GM pick-ups support. If there were not little green lights on the dash of these V8-powered trucks, you could not tell any difference whatsoever.
Rosario says, "We're operating the engine in two different states -- 4 and 8. We've gotten rid of the 6 because without a sophisticated mounting system it's difficult to isolate the disturbances of a V8 running as a V6. You could get a small fuel economy value from doing a six," he agrees, "but not right now. Someday it might be worth it, but not now." This system allows for "lesser kinematic complexity," Rosario says, "and a greater sophistication of electronics," which he calls a paradigm shift away from what other people are doing, most notably Mercedes-Benz with its variable-cylinder system on the new S-Class.