Frangible Fitting 737NG
The frangible fittings are in the retract hyd line for the MLG, when a flailing tread from a damaged tyre strikes the end of the fitting, it actually breaks off & ports the retract pressure to athmosphere, stopping the gears retracting.
It has to be a dual failure for it to function, a damaged tyre and a failure of retract braking.
The fittings protrude below the wheel well slightly, where they will be snapped off by flailing tyre tread, providing the retract braking has failed to stop the wheel rotating. all a bit agricultural.