You should flash your brake light to:

Question

Motorcycle riders should use their brake lights to communicate with other road users. It is particularly important for a motorcyclist to flash their brake light before slowing down to make a tight, fast turn off of a high-speed roadway. Motorcyclists should flash their brake lights when slowing in any location where other drivers may not expect them to slow down.

Accepted Answer

Tell other motorists that you are going to slow down or stop where they don't expect you to do so.

Answer

Tell other motorists that you are riding near them.

Answer

Help other motorists see your motorcycle while traveling in slow traffic.