In addition to the duty cycle determining the max amperage that the car can draw, there is a simple signaling protocol between the EVSE and the car which controls when the EVSE can apply AC voltage to the plug.
Basically, the EVSE produces a 1 kHz square wave at +12V into an open circuit when the car is not connected. When the car is plugged in, the car has a 2.74 kΩ resistor to ground which drops the voltage to ~9 V. When the car is ready to charge, it adds a parallel 1.3 kΩ resistor which drops the voltage to ~6 V. In addition, the EVSE can signal an error state by asserting -12 V. This is discussed in (Wikipedia).
The duty cycle of the 1 kHz pilot signal determines the maximum current the car is allowed to draw:
This is discussed in the same Wikipedia section (and the above table is from there).