I used to know these when I was a kid. Actually came in handy. Still use the count 1-10 on one hand technique. Here’s Wikipedia on the subject.
We BaRs were messing with it, and it is rather conducive to text speak. Since OMFG and WTF can be easily signed, as well as a host of other curse words and childish things. Yup, 40-something guys doing gang signs in text-speak. I guess you had to be there. But really, try it! Guaranteed fun!