On Uzi's they have a safety you have to squeeze as you pull the trigger, it's on the back of the trigger assembly and you have to squeeze it each time you pull the trigger or it won't fire. I read they put it there so the gun wouldn't accidently fire when paratroopers hit the ground. Of course that was on the open bolt SMG's but they carried that safety over to all the semi auto's that were imported into the country back in the 1980's before they got banned.