The Rules

Flatten is a mechanic that was released with set Bit Depth. It decreases the dimensions of opposing Digimon until they shrink into a point and vanish!  On its own, flat doesn’t do any damage or cause any immediate effects—instead, it’s a ticking time bomb. So here are the rules:

• When you meet your condition (in the pic, opponent used Circle, or opponent’s type is Dragon), you will cause a “flat” to the opponent
• On attack abilities, only one condition can be met per hit. (If a Dragon uses Circle, it only causes 1 flat, not 2.)
• Track “flat” by placing a marker to represent it in front of your active.
• If you would ever have 4 flat markers, you get KOd!
• If you would take damage if you haven’t attacked yet and have 3 flats, you get KO’d! (meaning opponent attacks first while you have 3 flats & you take damage)
• When you evolve by DP, you can subtract 10 DP from anywhere (such as your DP zone or your Immortalize card) to remove 1 flat marker. You can only remove 1 flat per evolution.

And that’s how it works. Check the rules for an official explanation (glossary or attack abilities). Flatten is specifically intended as an alternate win method, similar to trashing an opponent’s deck out (which may reduce your required KOs). You will still have to use all your cunning and experience to make the most of it, but it can be a powerful KO method. Since it “poisons” a Digimon, opponents may have to slow down their evolution progress. Or you could exploit them after they evolve to Mega for 2 KOs. Repeatedly gaining 4 flats can be difficult, even if you maximize your opportunities, so be sure to have a backup plan. You can get a KO sooner with flatten as long as you can hit first and supplement it with damage.

For more info on behind the scenes of this mechanic, I have a developer’s design diary article for Flatten.

Update 10-16-2020: Flatten rules now updated in this post to reflect the recent rule changes. Now requires 4 Flats or 3 Flats+Damage+1stAttack to KO; up from 3 and 2 respectively. Flats are represented by a token or marker instead of a card. Flats can only be removed when evolving by DP. “Paying DP” has been reworded to make more sense. Added more about the design and intent.