Rule 477.3.b
When an arithmetic effect from a source that is not a passive ability has a limitation that applies, it is limited at the time of its application, and is “remembered” at that limited level for the duration of its effect. This process is called “snapshotting.” Example: If an effect gives a unit “-4 [M] to a min of 1 this turn” choosing a unit with 2 [M], then the effect will generate -1 [M] this turn. Example: A unit reads “Units you control here have their Might increased to 5 [M].” This is a passive ability, so it will not snapshot. Example: A spell reads “Increase a friendly unit’s Might to 5 [M].” This effect is applied once, with an unlimited duration. Because it isn’t from a passive ability, it will snapshot.