Lexus GS430 User Manual page 28

QUICK REFERENCE
NOTE:
Vehicles with the cruise control: When the cruise control is being used, even if you downshift from "D" or "6" (sixth range) to "5" (fifth
range), or "D" or "5" (fifth range) to "4" (fourth range), engine braking will not be enabled because the cruise control is not cancelled.
For vehicle speed reduction, see page 400. Vehicles with the dynamic radar cruise control: When the cruise control is being used
in the conventional (fixed speed) cruise control mode or in the vehicle−to−vehicle distance control mode, even if you downshift from
"D" or "6" (sixth range) to "5" (fifth range) or "D", or "5" (fifth range) to "4" (fourth range), engine braking will not be enabled because
the cruise control is not cancelled. For vehicle speed reduction, see pages 423.
4