If it's bowed in the middle, hold it with both hands and just flex it in the opposite direction of the bow. Or, put one end of the shaft on a hard surface with the bend on top and and press down on it with the palm of your hand and work your palm back and forth on the shaft while pressing down on it. This will generate some heat fairly quickly as well. If the bend is particularly stubborn, or near either end of the shaft, you may want to heat it some before applying the pressure. Heat, put one end on a hard surface and press down, rubbing back and forth, with the palm of your hand over the bend. None of this compresses the wood fibers. If the bends still won't come out, then you need to work with some sort of burnishing tool like the commercially sold rollers or the shaft of a fairly large screw driver. This burnishing/wood compression tool takes the place of the palm of your hand. Can use it with or without heat. Just face the bend in the shaft up and press down on it with the burnishing tool and work it back and forth on the shaft over the bend. I think we have a tutorial on this site somewhere.