What's new about Legacies cards (compared to original We Didn't Playtest This at All cards) is they contain a slot for Owner and a picture. When you win a game of Playtest: Legacies, you claim ownership of the last card you played, by writing your name in the Owner box. FOREVER. If you own a card, you ignore it if it ever tells you that you lose.
Playtest: Legacies is a full stand-alone game (60 cards plus mystery packs). It may also be combined with the original We didn't Playtest This At All and its expansions.