what exactly does a 'no mod' patch do?
 

It sounds like it makes the particular ps2 release bootable without the aid of a mod chip or any swap trick....I am finding this hard to believe and am wondering what it's real purpose is. thanks.

Can't be done. The PS2 has to boot first and that is where the "secret" things are done by Sony to read pressed areas of the original game (that normal readers/burners can't reach) in order to check authenticity.

Certain modchips, like Messiah, fool the PS2 into thinking that it has properly read the CD/DVD and provided the correct booting data to allow the game to be loaded.