Historical Rules · 1 October 2009
Rule 615.1
Official source text from the 1 October 2009 Rules release
Some continuous effects are prevention effects. Like replacement effects (see rule 614), prevention effects apply continuously as events happen -- they aren't locked in ahead of time. Such effects watch for a damage event that would happen and completely or partially prevent the damage that would be dealt. They act like "shields" around whatever they're affecting.