I made a video a while ago showing what I call a "Kelly strap" for indirect percussion. I named it after a friend of mine who showed me how he was using it.
https://www.youtube.com/watch?v=ph1rfTWMlw4There are various types of indirect knapping. I came up with the "behind the knee" technique (or supported horizontal punch) but there are others:
shaft punch
unsupported horizontal punch
rocker punch
vertical punch
palm punch
You can find these techniques on youtube although it might be hard to find them because they may not have that terminology in the description.
Some Youtube creators who use indirect:
flintknappingtips (shaft punch)
jason newman
freeze cracked
hunt primitive (just recently started using it)
gregpryorhomestead
paleo greenByrd