Well, compressibility shifted the center of pressure of the airfoil aft - as transonic flow always does - and that brings the nose down, uncontrollably. The dive flaps shifted the center of pressure back forward so that the normal flight controls worked. They were not designed to either slow down the airplane (like Dive Brakes on dive bombers do) or magically make the airfoil more suitable for transonic speeds, is what I think you mean.