There is no direct mechanical connection (fixed gear ratio) between the Volt's extended-range 1.4L engine and the drive wheels.
Note how GM defines "direct mechanical connection" as "fixed gear ratio". However, to a technically minded reader this sounds like semantic squirming.
Here is how it works. There is a planetary gearset. In the Volt, the outer ring is locked by a clutch, and is also connected to the gas engine and generator motor. The generator can be used to recharge the battery, or to produce torque.
When the ring gear is locked ("grounded") to the transaxle case (C1 locked), there is no torque path from the gas engine to the wheels, which are connected to the planetary gears. If C1 is unlocked, C2 and C3 are locked, then torque flows from the gas engine to the wheels.
The point is, there is definitely a mechanical connection from the gasoline engine to the wheels, because the clutches can be commanded to send torque to the ring gear and therefore out to the wheels.