I've got that on my '09 Avalanche. It was standard equip on the 5.3 LS engine. Basically the computer shuts off the spark and injectors to 4 of the cylinders (no 6 cylinder step). It's virtually seamless when it happens. If I choose the computer readout on the dash I can watch when it switches mode, but other than that there's no feel to it. On my rig you pretty much need to be on dead flat terrain, or downhill, for it to be active. Throttle position is probably part of the data input the computer considers, but there are likely more data points that contribute to the calculations. I wouldn't imagine that fouling the plugs would be a consideration as there's no fuel to be burned when the cylinder is shut off.