
Basically, pulling on the ring will apply pressure to the trigger, which will loosen when the charging handle cycles back, resetting the trigger and when the charging handle travels forward again it'll tighten the string and pull the trigger again, thus allowing for full-auto fire.
Anyone wanna try this with an SKS or something with a reciprocating charging handle? I doubt you'll get any good groupings but it seems like a fun trick to do at the range.