Sweep is a rider comfort/control thing. Most riser bars have both a rearward and a slightly upward sweep when mounted in their neutral position. The angle chosen (6-9 rearward) is the average person's most neutral grip on the bars. Most bicycle bars are set sweep figures; but sweep is a very personal preference thing. A lot of motocross bar makers are starting to sell bends that are preferred by their star racers and will differ from normal production specs (Like, say, a Ricky Carmichael bend).