BMP resize is like any other patch, you can apply it to source code and then compile a build with new the new feature of BMP resize. Patches are not applied one at a time to the binary (rockbox.gigabeat etc.), but applied all at once to source code, and then a build is compiled for a specific device.
For an intro to patches, and how to build Rockbox see:
http://www.rockbox.org/twiki/bin/view/Main/SimpleGuideToCompilingThe patch allows the album art tag to give Rockbox instructions on how to resize the BMP for display
example:
%Cl|10|20|s150|s150|
%C
without BMP resize this would require all BMP's to be 150x150
what the patch does is put the code in place to understand what to do with the 's' characters, which are telling the WPS to scale the image to 150 (either shrink or stretch).
I have very little knowledge of C so I can't tell you exactly how the patch works in terms of computer code.