Historical Rules · 1 May 2012
Rule 614.1
Official source text from the 1 May 2012 Rules release
Some continuous effects are replacement effects. Like prevention effects (see rule 615), replacement effects apply continuously as events happen -- they aren't locked in ahead of time. Such effects watch for a particular event that would happen and completely or partially replace that event with a different event. They act like "shields" around whatever they're affecting.