Hartley All the Roads

It's difficult to get your pelvis rotated forward if you have a high saddle and poor hamstring flexibility. It's also difficult to do it if your bars are too high/close; rotating forward gives you a much longer effective reach since your back is much flatter.
The easiest way to learn how to do it is to sit on your bike without clipping in and rotate forward; you'll feel you're sat on a different part of your pelvis. You'll probably also find you have to bend your arms alot (which won't feel natural so you'll have to lower your bars).
Clipping in a pedalling in this position will take some getting used to, and if your setup isn't right then you'll probably find your body will revert back to it's original position. Lowering the saddle will probably help.
How hard was the gear you were pushing in the video?