[arijun]

What do you imagine happens if the battery is low and the car wants to start from a stop? The engine can't shift down to first so it would just stall out if you tried to use it to drive the driveshaft directly, and there's no other source of electricity to put into the motor driving the wheels.

[cwillu]

It's a planetary geartrain, with electric motors on the outer and inner, and the gas on the middle. If the electric is _completely_ dead, the gas will just spin without driving the wheels at all. There is no first gear, the mechanical advantage and “clutching” all comes from the inner electrical motor driving the sun gear, while the electric boost/drive comes from the electric motor on the drift shaft (which is on the outermost gear of the planetary set).

There's a video linked elsewhere in this thread that explains how the toyota system works, it's worth watching as it clears up several common misconceptions about the system.

[Toutouxc]

I think what they're trying to say is that there are regimes in which the HSD actually shunts energy from one electric motor to the other (i.e. one of them is "braking" and acting like a generator and the other one driving) and a big part (maybe the majority even) of all power goes through that path. This works even if the battery is empty, because the electricity isn't coming from the outside, it's coming from the spinning ICE.

[arijun]

Exactly. The vast majority of the launching torque comes from the electric motor, even if there is no battery at all.

[arijun]

> the gas will just spin without driving the wheels

... and instead drive one of the motors, generating electricity to power the other motor to push the car forward.

I feel like that video you're mentioning did a poor job if it didn't describe one of the more important and clever aspects of the Prius's design.