The breach phase is one of the three phases of Prismata gameplay. The other phases are the Defense Phase and Action Phase (which consists in Purchasing Units and Using Abilities, as well as the Breaching itself).
Defense Value vs Attack Value
Defense is a value calculated by the total health values of units with the blocker ability that are actively blocking. Note that units stop blocking when they use their ability or they have been chilled equal to their health. Attack value is simply the sum of the attack of the units attacking.
Wipeout and Breaching
If a player has Attack Power greater than or equal to the other player's Defense Value, the turn player wipes out all enemy blocking units, losing Attack Power equal to the health of all units destroyed in the process. After this, if the player may freely assign the remaining Attack Power as damage to any enemy units. This is referred to as breaching.
When breaching the attacking player must decide how to best assign attack to the opponent's units. Optimal play will depend on a number of factors, but consideration should be given towards low health attackers (such as Tarsiers) and destroying economic units (such as Drones).
It is notable that after wiping out or breaching, players are not allowed to unassign the sniping of a blocking enemy unit and freezing an enemy blocking unit can't be unassigned unless lethal damage has been assigned to that unit while breaching. However, the turn may be rolled back to a point before the breach was made with Ctrl-Z as normal.
Overkill is a mechanic that occurs when the attacking player has destroyed all active units and still has attack value remaining. Units under construction (normally invulnerable) will become vulnerable and be able to be attacked once all active units are destroyed.