fast explanation of how Swap disks cr4p works:
swap magic discs = SMD
1- the SMD have a program that just stops the disk from spining, and have 2 options "start the game again" = LOAD program, and some other tweaks for change the video mode.
2- once the disk stopped spining, and you have the cd-tray door removed, you can now use the slide card to bring out the cd-tray.
3- the slide cards moves a lock and slides the cdtray out.
4- now .you have the cd-tray opened without pressing the eject button. ( if you presed the eject button, when it closes again the ps2 will try to check if the new disk inside is a valid ps2 disk, from the same region US, Japanese, PAL, etc. . but since you didnt press the eject button the ps2 didnt notice the cd-tray was open, then it will not check again if the cd inside is valid)
5- swap the disk with your original GT4P Japanese.
6- manually close the cd-tray pushing it inside. then move the lock back to its position with the slide card.
7- select on the screen "LOAD PROGRAM" this command make the ps2 runs the current cd from the begginign, but without checking if its a valid disk
8- PLAY!