A simple mechanical device that is fitted primarily wherever there is a risk of a ruptured hydraulic line causing total loss of fluid so they are typically found in the brake lines and u/c tilt actuator lines (if fitted!).
If there is a sudden high flow of fluid a spring loaded shuttle moves across to shut off the flow (sorry can't think of a better description at the mo')
The 747 also had them fitted in the hydraulic lines to the empenage after the failure of the rear pressure bulkhead and fin on the JAL a/c that was lost.