The  reason these numbers are important is because of the correlation  between ball reaction and high Ra and Rs values. Essentially, the  greater the Ra and Rs number, the more friction the coverstock can  generate. The more friction the coverstock can generate, the greater  hook potential the ball will have. What happens when a ball dies is the  coverstock has lost the surface deviations originally found on the ball  from the out of box finish. These deviations (high Ra and Rs numbers)  are created in our finishing and rounding process by creating deep  grooves, cuts, and scars on the surface of ball. Later, when the ball is  finished, these rough surface deviations are smoothed and ‘polished’  but they aren’t completely removed. If you were to then examine the  coverstock on a micron level, the steep peaks and valleys would still be  on the surface of the ball, but they would be rounded. This creates the  type of ball motion where the ball will still skid through the heads  and midlanes cleanly, but still have a tremendous amount of friction  generation capabilities as the ball enters the buff or exits the  pattern."

 

Hope that helps.