And if you're moving faster than 7 or 8
MPH, the gas engine NEVER ahuta off. This is different than in a Prius or Escape. What happens is that (on occasion) the intake valves shut off the gas flow to the engine, but the crankshaft keeps turning. Therefore, engine on, gas off. It's cool to know, and it occurs occasionally just naturally as you drive. You CAN induce it, like Harold explains, but I do not strive for it often. Using up the IMA's juice is cool (as you may not be using any gas at that point), but you must replenish the battery juice you used up - and it's not 100% efficient - so the cost is almost always higher than the gain.
Trying NOT to use the battery pack, except when it's needed for extra acceleration or hill-climbing, is the best way for the typical driver to get better MPG.