One of the bores in the shift tower (left) is deeper than the other. JB Conversions installs a check ball under new stiffer springs so they sit at the same height. This places an equal amount of pressure on the two shift rails, eliminating the tendency for the rear shift lever to jump out of gear when the case is twin-sticked. Lower spring pressure is desired on the inner rail when using the J-linkage intended for a single stick. This lower spring pressure is needed to allow the cam action to work on the single stick linkage plate. JB sells the spring and ball kit individually, as well.