What is the LD_PRELOAD trick?
If you set LD_PRELOAD to the path of a shared object, that file will be loaded before any other library (including the C runtime, libc.so). So to run ls with your special malloc() implementation, do this:
$ LD_PRELOAD=/path/to/my/malloc.so /bin/ls
You can override symbols in the stock libraries by creating a library with the same symbols and specifying the library in LD_PRELOAD.
Some people use it to specify libraries in nonstandard locations, but LD_LIBRARY_PATH is better for that purpose.
With LD_PRELOAD you can give libraries precedence.
For example you can write a library which implement malloc and free. And by loading these with LD_PRELOAD your malloc and free will be executed rather than the standard ones.