The high - profile camshaft lobe which normally contacts the secondary exhaust rocker arm alone during low - RPM engine operation is able to move both exhaust rocker arms together which are locked as a unit. (en.m.wikipedia.org)
When cruising along or idling at a stoplight, the system engages a set of camshaft lobes and can keep the valves open a bit longer to reduce pumping losses and help economy. (articles.orlandosentinel.com)
Their analysis was that it was a mechanical engine problem, like piston rings, flat camshaft lobe, or value issue. (mechanics.stackexchange.com)