> How does it negotiate with a host-powered device if it's unpowered to begin with?
Through a pair of resistors.
The unpowered device connects each of the two CC pins to the ground pin through a separate resistor of a specific value. The cable connects one of these CC pins to the corresponding pin on the other end of the cable (the second pin on each side is used to power circuitry within the cable itself, if it's a higher-spec cable). On the host side, each of the two CC pins is connected to the power supply through a separate resistor of another specific value. When you plug all this together, you have the host power supply connected to ground through a pair of resistors, which is a simple voltage divider. When the host detects the resulting voltage on one of the CC pins, it knows there's a device on the other end which is not providing power, and it can connect the main power pins of the connector to its power supply.
Through a pair of resistors.
The unpowered device connects each of the two CC pins to the ground pin through a separate resistor of a specific value. The cable connects one of these CC pins to the corresponding pin on the other end of the cable (the second pin on each side is used to power circuitry within the cable itself, if it's a higher-spec cable). On the host side, each of the two CC pins is connected to the power supply through a separate resistor of another specific value. When you plug all this together, you have the host power supply connected to ground through a pair of resistors, which is a simple voltage divider. When the host detects the resulting voltage on one of the CC pins, it knows there's a device on the other end which is not providing power, and it can connect the main power pins of the connector to its power supply.