The Echo Trigger is a revolutionary new pull and release trigger from FosTecH that has a groundbreaking drop-in design that is approved by the ATF. This is the Gen II Echo Trigger and does not require the proprietary bolt carrier body in order to function like the original Gen 1. Works with any standard full-auto style bolt carrier group which 98% of all rifles currently come with. The Echo Trigger allows the shooter to select from 3 different settings: Safe Mode: Firearm will not fire. Semi-Auto Mode: Firearm fires when the trigger is pulled. Echo Mode: Firearm fires when the trigger is pulled and also when the trigger is released.