DJMAX RESPECT V

Every time you load a song, if it has a hidden BGA (Background Animation) there's a 20% chance it'll show instead of the default background that normally plays during that song. There are 6 songs in the base game that have hidden BGAs, which are:

[Portable 2] A Lie
[Portable 1] Enemy Storm
[Respect] NB Ranger - Virgin Force
[Portable 1] Never Say
[Portable 1] Out Law
[Portable 2] Whiteblue

If you don't get the hidden BGA for the song, hitting Restart will not cause the game to roll for the 20% again, the background will stay the same across restarts. You need to exit to the Music Select and load the song again from there.
If you don't know what the hidden BGAs looks like, here is a video that shows off the differences between them. The left is the normal BGA, on the right is the hidden one. It should be noted that you don't need to clear the song once the BGA has loaded, after you've confirmed you got the hidden BGA you can just quit to the song select and do resets for the next one, the achievement will pop once the 6th BGA has started playing. If you're aiming for the most in-game unlocks you'd probably want to Max Combo with the hidden BGA playing so you have it unlocked it in your collection.