When I have seen it done in videos or shows they use a flat piece of wood with a hole drilled in it a slight bit larger then the shaft with the edges properly chamfered or smoothed. The flat isn't very thick. I think experience is the guide here. The shaft is then warmed over a fire, careful not to scorch the wood. Then the shaft is place thru the hole and the flat board is used as a lever to bend the shaft. Most of it is the skill of experience here and time I believe. Patience is a big part I imagine also. But then when you live in the jungle and your next meal depends on a straight shaft patience is easier to find.