OK, maybe it' s too late for me to be trying to make any sense out of what all has been wrote, but it seems the comments have jumped all over the place. If it is too late for me to read it is probably too late for me to comment, but here goes.

The brace height is measured from the " Throat" of the grip to the string at rest. The power stroke is measured from the string at rest to the string at full draw. Your draw length is measured from 1 3/4" in front of the " throat" of the grip to the string at full draw. If your brace height is 7" and your draw length 30" , then your power stroke is 21 1/4" .
If that brace height is decreased by 1" , your draw length remains 30" but the power stroke has just increased by 1" . When a manufacturer chooses to build a reflexed riser it is in order to decrease the brace height which in turn increases the power stroke. This increase in power stroke increases speed, approx. 10-12 fps/ inch of power stroke. The longer the power stroke or shorter the brace height, one in the same, the more time the arrow is on the string. Therefore, the speed increases due to more transfer of energy to the arrow and there is more likelyhood of error on the shooters part because of this extended time on the string.
The newer bows with the parallel limb designs do use a reflexed riser to increase power stroke. Due to the parallel design the brace height would be increased dramatically and the power stroke decreased yielding a slower arrow. This would mean a more stable bow, but their intent was to design a fast shooting bow that had a minimal amount of vibration and in so doing they have incorporated a parallel limb design to reduce vibration and increase stability and at the same time reflexed the riser to maintain the speed that most shooters seem to demand.