I have been really working hard on my Double Underhand Bending over the last few months and it is finally coming around. Today I went after a piece of very tough 5/16" X 7" 0-1 Drill Rod. This was pretty tough 5/16 and I would estimate it would rate in the 475 - 480 lb range. I didn't quite get it to 40 degrees, but that will come soon enough. My ultimate DU goal is a 5/16 X 7" Grade 5 bolt (Edgin).
There have traditionally been two styles of double underhand bending: traditional and modified. Traditional is with the wraps fairly close together and the bar travels from neck level down toward the waist. The bar is mainly bent with wrist strength. Modified style is basically an upside down double overhand bend. The bar remains high and with a double underhand grip you push in on the ends like you would with a DO bend. This style incorporates the larger shoulder and chest muscles and typically allows people to bend much bigger steel.
My style is a traditional / modified hybrid. I incorporate the motion and technique of a traditional bend, using mostly wrist strength to bend the bar. Where I deviate from the traditional style is that I move the wraps further apart. Typically I put them 1.75" to 2" from the ends.
Take a look at the video below to see my technique and give it a try. Whatever double underhand technique you use, I would encourage you to incorporate DU into your training as it is an excellent way to strengthen your wrists and become a complete bender.