most of the time when I do it, I cut a piece of pipe, that the square will just fit into, to the length that I want the twisted section.
then I have old sockets welded to a plate with a hole through the center, that holds one end of the square, then slide the pipe over the square, then a big tap handle for twisting it...if you keep the pipe tight to the socket, and the tap handle tight to the pipe, it stays pretty straight...as you twist it the square stock becomes shorter pulling the tap handle/pipe/socket together, and as long as the pipe is cut square it keeps everything straight...
long twists do bend a lot more than something like the short twist on the poker...