Not everybody has the "A" back style Cobb talks about here. He does a video for what he calls the "B" back style with a chunkier rider.

I'm 60y.o. and don't ride as aggressive as this video by a long chalk but I do have my hips rotated forward enough so my back is flatter and weight is off my sit bones. I am probably more of a "B" style back. I started riding like this in my early 40's and it made a lot of difference in my long distance comfort. It did take some getting used to and some effort to change my riding habit.

Something you have to realize about this style is that you use some of your back's inflexibility as kind of a spring to take weight off your hands. It's your lower back that will really talk to you at first when you get this right but as you adapt it gets real easy.