A technique to lock the decoder to prevent accidental programming or to enable programming two decoders in the same locomotive separately which share the same address. This allows one decoder to be locked, defeating the ability to program it, while allowing the other to be programmed. This feature can also be used to prevent accidental programming of a multifunction decoder.
This feature was used when a multifunction decoder was installed in a locomotive which did not support sound, or if the modeller desired more functions than it supported. An additional decoder, such as a sound or function decoder sharing the same address, could be added and programmed independently.
Today it would be rare to see such an arrangement, as multifunction decoders support a wide range of functions and sound effects.
For more information see the Decoder Programming Lock page.