Quick-connect plug-in couplings, with shut-off facility at both ends, work in accordance with the 'push & pull' principle and are also ball locked. The locking sleeve is fixed in the locked position by a pre-tensioned spring and can be pushed axially in both directions to enable connection and disconnection. These design characteristics make this coupling type suitable for use as an emergency breakaway coupling. When combined with the breakaway feature, the coupling is connected by simply pushing the plug into the socket and disconnected by pulling out the plug. Straight through plug-in couplings have no valves. They are actuated via the locking sleeve. This type corresponds to the quick-connect plug-in couplings.