The system is based on two elements:
The underwing coupling is closed by a spring-loaded lid which can be pushed up to open the tank fuel line.
The refueling nozzle has a poppet valve which interrupts the fuel flow and when moved out allows the fuel to leave the nozzle, and pushes the tank coupling lid, allowing fuel to flow to the tank.
This is the general design and mode of operation:
Now if we focus on the nozzle:
Source (fuel exits at the bottom).
The poppet valve is at the level of the red annular bumper, and looks like this:
The poppet has number 15 below, and is resting on seal 43 until the nozzle is locked:
This ensures fuel cannot exit the nozzle prior the nozzle is locked, and after it is unlocked:
Handle 14 is used to lock the system and open the poppet with link 34 and shaft 32.
See that in action on this video. Note how the nozzle is first rotated with the ring handle after it is hung to the underwing intake, and then the smaller locking handle is operated to allow the fuel to flow. The same is done when disconnecting the nozzle, in reverse order.