For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.

To determine if you are way behind, check the game state as it was at any point during the current turn, regardless of whether Knight of Lost Causes was on the battlefield at that time. For example, if you control one creature and an opponent controls four, then you play Knight of Lost Causes going up to two creatures, you are way behind this turn because, at some point in the past the turn, that opponent controlled three creatures more than you.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as and . These modified symbols should be treated as the standard symbols during play.