I've watched a few of those on YouTube. I've done a fair bit of mountain biking, although not DH, more XC (over the mountains round where Germany, Austria and Switzerland meet, or Lake Garda), and it blows my mind what these guys do. I'm glued to the screen from start to finish.
Ironically I've just noted that the acronym of downhill urban mountain biking is DUMB. I would need a full frontal lobotomy to throw myself down a course like that. Not to be misunderstood, I mean I'm struck dumb by the level of skill, not to mention cojones these guys possess.