...one I'm replying to now (#140147) was the first time I ever heard it described.
AFAICR.
OTOH, as a kid, I used to throw apples -- the sour little ones that grew on our old trees weren't really good for much else, most years -- by spearing them onto the point of a springy stick and them whipping them away, much like you describe the "German slingshot" does with rocks. (Down to sharing the same main weakness, namely that the shooter can't exactly determine the instant of release.) Made quite impressive splashes on impact... :-) (And was probably a lot less dangerous, both to property and people {not that I ever aimed at any... much! ;^} than the rocks you mention.)
I can't remember whether I came up with this stunt all on my own... It's quite possible that I got a tip from my Dad; and if it happened that way, then it's not just possible but likely that he, in turn, was adapting the "German slingshot" a bit. (Probably in order to save his daughters' lives. ;^)
I'll try and remember to ask him, when I talk to him next. (i.e, don't hold your breath... :-/ )