Most hybrids have continuously variable transmissions, or CVTs. CVTs change gear ratios in a continuous, fluid fashion rather than stepping up and down through, say, four or five gears. That's ideal for maximizing fuel economy, but CVTs lack strength. They can't handle big V8-engines, and they aren't good for towing.
The Chevy Tahoe has a four-speed transmission, but with the interaction of those electric motors, it can act like a CVT. When fixed gears are better for towing a heavy load, the electric motors change their behavior and allow the transmission to act like a regular four-speed.
The engine has its own suite of fuel-saving touches. At 6.0 liters, it's bigger than the 5.3-liter engine that's standard on the Tahoe. That may sound backwards, since the goal is fuel economy, but this engine can shut off four cylinders when full power isn't needed. With a bigger engine, half-power is enough more of the time.
