I'm not much of a mtb'er, but I've been wondering about this behind-the-saddle position myself. There's a heinously steep slope down one of the paths I sometimes ride to work and I have to get all the way behind the saddle to not topple over forwards. It helped when I switched my saddle to a narrower, shorter one, but it does feel very strange to be "hanging" from the bars and not steering with my thighs anymore, I'm obviously used to doing that a lot. I'm a little terrified of sitting down on my rear wheel...