TOYOTA 2011 4Runner - Manual (Page 202)

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2-1. Driving procedures
■ Downshift restriction warning buzzer (S mode in multi-mode type)
To help ensure safety and driving performance, downshifting operation may
sometimes be restricted. In some circumstances, downshifting may not be
possible even when the shift lever is operated. (A buzzer will sound twice.)
CAUTION
■ When driving on slippery road surfaces
Do not accelerate or shift gears suddenly.
Sudden changes in engine braking may cause the vehicle to spin or skid,
resulting in an accident.