I'd file this gizmo under "something I don't know I didn't need". It’s a unit that attaches to the rear brake light and activates it when it detects deceleration. It therefore operates the light under engine braking as well as, eh, brake braking. I don't feel that engine braking on any of my bikes is such that I need this. But if you do see here
I wonder if this is strictly legal in the UK?
Fitting & test video
This Slovenian company also does a self cancelling indicator system (didn't Yamaha have this in the '70's?)