I came across a reference to it recently on proggit and (as of now) it is not explained.
I suspect this might be it, but I don\'t know for sure.
You can override symbols in the stock libraries by creating a library with the same symbols and specifying the library in LD_PRELOAD.
LD_PRELOAD
Some people use it to specify libraries in nonstandard locations, but LD_LIBRARY_PATH is better for that purpose.
LD_LIBRARY_PATH