What does this driver indicate by moving his arm alternately up and down with short, rapid movements?

Question

DentroTest · Accepted Answer

The intention to immobilize the vehicle or to brake its progress considerably must be warned whenever possible. This warning is usually given by repeatedly using the brake lights. Failing this, or as a complement, it can be signaled by moving the arm alternately up and down with short, rapid movements. In this way, drivers behind are warned to avoid rear-end collisions.

What does this driver indicate by moving his arm alternately up and down with short, rapid movements?

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