In the kickdown position a valve is openned in the trans that dumps governor pressure (or some such). This activity superceeds the normal balance of control pressure against governor pressure on the shift valves.
When one's foot is not on the floor the shift point is determined by the control pressure mostly and somewhat by the modulator pressure. In other words shift point sare regulated by throttle position until kickdown. At kickdown the shifts are determined electrically by rpm.
Bosch Master, ASE Master, L1
33 years MB technician