Constrict is an extraordinary ability with the descriptor
A creature with this special attack can crush an opponent, dealing bludgeoning damage, after making a successful grapple check. The amount of damage is given in the creature’s entry. If the creature also has the improved grab ability it deals constriction damage in addition to damage dealt by the weapon used to grab.
Extraordinary abilities states:
Using an extraordinary ability is usually not an action because most extraordinary abilities automatically happen in a reactive fashion. Those extraordinary abilities that are actions are standard actions unless otherwise noted.
Is constrict considered an action and thus takes a standard action to use, or is it "automatic" and reactive, thus not taking an action?