This is a special Flex cable that is able to convert 9V to 18V. In the middle of the cable which is 50cm there is a built-in step-up converter which doubles the voltage. When plugged in a 9V input you would have 18V on the other side. It is able to power up a single 18V pedal and its maximum output current is 50mA.
Be careful when using it with the Double Jack. When using one of the 12V outlets with the Booster Flex you will end up with 24V on the DC-plug and that might damage your 18V pedal. So stick to 9V outlets when using Booster Flex.