Short story: He has propably used ATA command called "secure wipe". That uses hdd ATA specs/feature, that sets the drive to wipe itself.
Long story: When you set ATA password, you can unlock it only by either typing the ATA password or using ATA secure wipe command to wipe the entire hdd. If you interrupt that progress, it will continue to ask password and continue to wipe the drive if none is given. This feature is set so that you could not simply reset the ATA password, but only to do so by first wiping the entire hdd.
ATA passwords are no means secure way to protect hdd, but they do keep out most criminals from accessing the content of hdd. Some BIOS support ATA security commands, most dont.