"Where mountain bikers get into trouble is steep descents, where lack of skill or discretion (i.e. the sense to get off and walk) lead to locking the back wheel and sliding down."
The downhill part I instinctively understand, but how do they manage to get up steep ascents without losing traction on dry loose stuff and starting to throw rocks, dirt, etc? Or do they sometimes just have to get off and walk up as well? I've watched a lot of MTB's out on the local logging roads when I was still running. They can get pretty steep at times, and the bikers generally weave back and forth up the road, very slowly, to reduce the grade. I have often wondered how they would handle a narrow, equally steep trail, where weaving back and forth is not an option.