ITs easy, well first you make a multibios using two 512k bioses. I have is so my first is original ms bios and second is X2. Since xbox boots from the last 256k, when you flash you still boot from x2 so you don't have to worry about not booting to a hacked bios after flash.
What you wanna do now is to limit the tsop to 512 so when xbox boots, it loads the second 256 in the first 512 which in this case will be the original bios or whatever else you want. The switch goes on a19 and ground.
here is a picture that shows you where a19 is located. This tutorial was posted by ^101 before somewhere on these forums, so i want to give him all the credit. http://www.xenatera.com/bunnie/proj/anatak.../rompinouts.jpg