The only function of
the kickdown relay is to ensure that the transmission shifts instantly and
reliably at redline. The relay receives an RPM signal from the ECU at redline.
The relay activates, opening the normally-closed contacts. This breaks the
signal from the kickdown switch under the throttle pedal, causing the
transmission to upshift instantly. As soon as the transmission shifts, the RPM
signal disappears, the relay deactivates, the N-C contacts close, and you are
back in kickdown mode (eager to downshift, reluctant to upshift).