Because Weer'ds comment section is malfunctioning...
Photos taken of pages from Small Arms of the World 12th Edition, Ezell (1990).
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How this all works... In semi the trigger rotates, taking the sear with it and that releases the hammer. The sear pivots on the same pin as the trigger, but is slotted. When the nose of the sear is caught by the hammer, it's pushed back and the tail of the sear rests above a shelf on the trigger. It's this shelf that raises the tail of the sear, thus lowering the nose and releasing the hammer. As soon as the hammer is flying, the sear moves forward and the tail drops down into a groove in the trigger and raises the nose to catch the hammer as the bolt-carrier resets it; which shoves the sear back into the groove so you have to release the trigger to let it slide on its slot and put the tail over the shelf again.
The safety sear tries to catch the hammer every shot. When the bolt carrier locks the bolt, the rest of its movement pivots the safety sear and releases the hammer. In semi-auto mode, that lets the sear grab it, but in auto, the trigger can pivot a bit more and the nose of the sear is held too low for it to grab the hammer; it's the safety sear releasing that sets off the next round.
Military FALs which were "semi-auto-only" have a block on the selector on the outside of the gun. Inch pattern guns have a lobe that will hit the upper receiver preventing it from rotating all the way to the auto position.
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Metric guns have a hook that engages a pin on the lower that prevents the same movement.
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The shaft of the selector in both cases is still cut with the extra depth that lets the trigger move the nose of the sear below the notch of the hammer. As you can see, a couple of minutes with a grinder or Dremel will restore the auto-mode.
In the US, having the safety sear, or even having the upper cut to accept it, makes your FAL a machine-gun unless you have one of the very few "sear-cut" guns FN imported under the Browning imprint.
Photos taken of pages from Small Arms of the World 12th Edition, Ezell (1990).
How this all works... In semi the trigger rotates, taking the sear with it and that releases the hammer. The sear pivots on the same pin as the trigger, but is slotted. When the nose of the sear is caught by the hammer, it's pushed back and the tail of the sear rests above a shelf on the trigger. It's this shelf that raises the tail of the sear, thus lowering the nose and releasing the hammer. As soon as the hammer is flying, the sear moves forward and the tail drops down into a groove in the trigger and raises the nose to catch the hammer as the bolt-carrier resets it; which shoves the sear back into the groove so you have to release the trigger to let it slide on its slot and put the tail over the shelf again.
The safety sear tries to catch the hammer every shot. When the bolt carrier locks the bolt, the rest of its movement pivots the safety sear and releases the hammer. In semi-auto mode, that lets the sear grab it, but in auto, the trigger can pivot a bit more and the nose of the sear is held too low for it to grab the hammer; it's the safety sear releasing that sets off the next round.
Military FALs which were "semi-auto-only" have a block on the selector on the outside of the gun. Inch pattern guns have a lobe that will hit the upper receiver preventing it from rotating all the way to the auto position.
Metric guns have a hook that engages a pin on the lower that prevents the same movement.
The shaft of the selector in both cases is still cut with the extra depth that lets the trigger move the nose of the sear below the notch of the hammer. As you can see, a couple of minutes with a grinder or Dremel will restore the auto-mode.
In the US, having the safety sear, or even having the upper cut to accept it, makes your FAL a machine-gun unless you have one of the very few "sear-cut" guns FN imported under the Browning imprint.