Vibram Flight Guide and Ratings
The top speed number is the ideal launch speed (in mph) of the disc. Thrown flat at this speed, the disc will hold the line. The bottom speed number is the distance in feet the disc will fly when optimally thrown.
At a speed of 20% less than the ideal speed, this is the flight angle of the disc. For example, the VP - ideal launch speed of 60 MPH - at 48 mph, the disc will be flying 24 degrees left. (assuming a RHBH).
At a speed of 20% more than the ideal speed, this is the flight angle of the disc. Again for the VP, at 72 mph, the disc will be flying 2 degrees right. (assuming a RHBH).
There are times when you want your disc to turn left or right. Throwing a disc faster or slower than its ideal launch speed will get this result.