In all automatics there are a number of different fluid pressures that are separate systems within the trans. The oil pump (or 2 if used) will supply unregulated pressure to the pressure regulator. From there these systems use different pressures for different jobs. For example, it takes more pressure to hold certain geartrain items in reverse than forward gears, so that "Line" pressure will be increased in reverse gear to allow bands or clutches not to slip. "Throttle Pressure" is what the kickdown linkage is involved in. At light throttle input throttle pressure is lower, and this helps to create normal, softer shifts for passenger comfort. At higher throttle openings, increased engine torque demands the clutched and bands to be applied faster and with with greater pressure to lower slippage between shifts. The whole key to this is "throttle pressure" increases with more engine load. Governor pressure is another type of fluid pressure that dictates when the transmission will upshift or downshift, dependent on engine load (directly affected by throttle pressure). "Kickdown Linkage" actually is referring to this same mechanical connection from the carburetor to the throttle pressure valve inside the transmission. Although we commonly refer to this as "Kickdown Linkage" (and it does have a say in kickdown) it more correctly should be addressed as the connection that controls throttle pressure. If there is no linkage installed an internal spring on the valve body will force the internal linkage to return to a kind of "idle" position, making the trans think there is very little or no throttle being applied. So if the trans "thinks" the engine is under low or no load when in fact it's at full throttle, you can see how the clutches/bands would be under heavy load with insufficient throttle pressure to prevent slipping. That in a nutshell is why we need that linkage. If you have a full manual valve body modifications to the valve body eliminate the need for this linkage.
