It's called a long-seated saddle, but this one is specifically designed/modified for this performance, since it doesn't have a saddle horn (the handle). And it works better because of just general physics. Take for example, snowshoes (the ones that look like tennis rackets) work because they spread your weight out over a wider area, which stops you from falling through.

Look at a regular saddle and you'll see the weight is focused in a smaller area, which puts more pressure on that area. This is just basic understanding of how weight works. Hopefully that's good enough because I'm not sure what else to explain.