The 12V is not coming from the PSU. It can come from the battery positive terminal or an empty appropriately-rated fuse slot that provides power all the time. The "Vacc" is coming from an ignition-switched power source, such as an empty fuse slot that is powered on/off by the ignition key state. The variable resistor is showing you can use varying resistors to adjust the brightness of the LED.
One thing that doesn't look right is how he's got the main 12V lead going to both the relay and the coil. Also, the relay's not grounded, which I believe relays usually are.