Board Thread:General Discussion/@comment-28126346-20160404034624/@comment-27701762-20160404045711

The only thing that is remembered by the game is that you completed a Genocide run. When you perform a True Reset, the rest of the game reverts back to a state that is essentially as though you are playing it for the first time.

Removing the flag requires a solution outside the game, through manipulating the game's files. Alternatively, you can uninstall and then reinstall the game (though if you're playing the game through Steam, you need to turn off Cloud saving).

The effect is something that I think should not be revealed. You should go through blind with regard to the effect, and find out for yourself what happens. Hence why I haven't specified.