Yep you're right, well with Ford anyway, they call it capless refuelling. In the top of the neck there is a metal sprung loaded plate which is locked in position, when you put the the diesel nozzle into the neck (almost an interference fit) it pushes two lugs apart (situated approx 4 and 8 'o' clock) that then releases the lock on the metal flap and the nozzle goes all the way in.
On the downside unless you have got the special cheap and nasty plastic funnel that Ford supply if you run out of diesel you are fcuked as you can't put any fuel in the car.
How do i know all this you ask, got a new Fiesta for the wife and guess who ran out of fuel. After going backwards and forwards a couple of times from the car to home i improvised with a 1/2 inch drive deep socket (17mm i think) then passed a radiator overflow pipe down the middle of the socket used an off cut of fuel pipe and hose pipe to get a large enough diameter to put a funnel into and drizzled a couple of gallons of diesel into the tank which took forever.
Technology and innovation, don't you just love it.