The selector switch has as it's uppermost position, the "safe" position. The selector's spindle has cutout slots which prevent the trigger from traveling a sufficient distance in the safe position to allow the sear to disengage from the hammer notch. The selector, when in either the semi-automatic or full automatic position allow the trigger to trip the sear allowing the hammer to fall.
The difference between the semi-automatic position and the full-automatic position is the selection of a secondary sear mechanism. This mechanism, when engaged, will also allow the hammer to fall from it's fully cocked position when tripped by the safety sear lever. This lever is engaged by a ramp formed by the absence of a cut on the bolt carrier's lower face. When the bolt carrier has reached its full forward position (and the bolt is thus fully seated), the ramp on the bottom of the bolt carrier will have moved the safety sear to it's trip position allowing the hammer to again fall (unless prevented by the primary sear controlled by the trigger position).
The following illustration shows the trigger group when
the selector switch is in the "burst mode" (F) position.
This mode of operation allows continuous firing until
either the ammunition is expended or the trigger is released.
The next illustration shows the trigger pack when
the selector switch is in the "single fire" (E) position.
The trigger group is removed from the pistol grip
housing by first rotating the fire selector lever
upwards until the lever is vertical (as shown
in the following figure).
After the fire selector lever has been rotated, it is
simply pulled from the assembly.
Then the trigger group is removed from the
pistol grip housing.
The type of trigger group shown is what is known in the United States as a "Registered Sear" style. The component of this type of machine gun is the single part known as the "sear". This part is located in the trigger group frame (as pictured) in the lower left hand corner of the assembly. The sear is held into the assembly by the pin which is visible (again the pin in the lower left hand "corner" of recessed notch of the assembly).