Weapon Triangle / Break
Weapon Triangle returns along with a new mechanic.
After sleeping through Three Houses, the Weapon Triangle makes its return in Fire Emblem: Engage. It's mostly the same old rock, paper, scissors that fans have come to love (or not): Swords beat Axes, Axes beat Lances, Lances beat Swords; except there's also the addition that Arts now beat Bows, Knives, and Tomes.
Unlike previous iterations, having weapon triangle advantage or disadvantage does not affect combat parameters at all. Atk/Hit/Crit all remain the same, regardless of whether you're facing weapon triangle advantage, neutral, or disadvantage.
If a unit lands a damaging attack on their turn while having weapon triangle advantage, they will inflict the new Break debuff on the enemy. Player unit counterattacks on enemy phase and vice-versa don't count, and neither do 0 damage attacks.
Having the Break debuff inflicted on a unit is essentially that unit dropping their weapon, and they're too shocked to pick it back up again until after they get attacked, or when their next turn starts (where the Break debuff wears off). While afflicted by Break, a unit cannot make an attack, including any counterattacks that would have happened in the round of combat where the Break occurred. Break always wears off at the start of the affected unit's turn, so this is a Player-Phase-only benefit.
Certain factors can cause units to not care about Break, and hence the weapon triangle all together. Most notably, units in an Armored class cannot be affected by Break, which means they will always be able to get counterattacks in (range dependant). There are some skills that interact with the Break mechanic as well, for example Emblem Marth's sync skill Break Defenses.