Read this -> https://www.bevfitchett.us/heckler-k...inciple-1.html
In semi auto mode, the auto sear trip must be released first. The hammer falls slightly and is engaged by the disconnector which then holds the hammer back until the trigger is depressed. The hammer then falls igniting a cartridge.
In full auto mode, the disconnector is held back by the trigger and cannot be engaged. When the auto sear trip is released by the bolt carrier, the hammer falls completely igniting a cartridge.
As an aside; on an HK machine gun the auto sear always engages the hammer even in semi auto mode. By contrast on a real M16, the auto sear can never engage when the selector is set to semi mode. The safety moves the auto sear out of possible hammer engagement. An AR-15 DIAS (Drop in Auto Sear) works exactly like an HK weapon. The auto sear is always engaged even in semi auto mode.