I think you are referring to a "bench hook"
http://www.carvingpatterns.com/brace.htm
A good and easy/cheap way of securing a board for relief carving. There are other ways of clamping a board down and depending on size and accesability there may be better ways, but these bench hooks serve really well for reasonably sized reliefs.
I've also found that the "bench dog" type clamps work really well, and there are a couple vaiations of these with scrw clamps that i really like!
Al