As Justin said, the intent is to demonstrate the abiltiy of the maker to differentially heat treat the blade. No where does it state that the blade must come back to true. The ABS test guidelines state:
4. BENDING: THE PURPOSE OF THIS TEST IS TO SHOW THAT THE APPLICANT IS ABLE TO HEAT TREAT A KNIFE WITH A SOFT BACK AND A HARD EDGE.
"The applicant will then bend the blade ninety (90) degrees. The supervising Master Smith will signal the applicant when the ninety (90) degree angle has been reached. The blade is allowed to crack at the edge on bending but not beyond approximately one third (1/3rd) the width of the blade leaving two thirds (2/3rds) of the blade intact. However, if any part of the blade chips or any part of the blade or tang breaks off, the applicant fails. Because of the many variables in the size, geometry, and temper line of the blade, the Master Smith using his/her judgment, shall determine if the extent or location of the fracture line is acceptable."
I have omitted a good portion of Step #4 guidelines to only show the exact information. Keep in mind that it does not state that the blade must go back to true. All of the test blades I have seen have a bend remaining in the blade after testing. I did hear rumor of a batch of 5160 which the ABS school recieved not long ago. It was used during class and a couple of students used it to produce thier test blades during class. I was told that the blades not only passed the the bend test, but the blades returned to true. Like I said, I was told this and I can't remember the source, therefore I will call it rumor.