[mc3301]

'avar' is talking about "engine braking" with a manual transmission car. Most motorbikes have a manual transmission.

If you gear down (you are cruising in fourth gear and 'gear down' to third gear), your vehicle's speed will reduce.

Brake lights are triggered by the brake pedal or lever/pedal on a motorcycle.

In the case of engine braking, the brake lights are not triggered. Drivers behind them can't anticipate stopping without such a signal.

Try it yourself! Some automatic cars, you can gear down, or just pull your parking brake a little. Cars behind you will come dangerously close to hitting you. Also, don't try this.

Wait, do new cars' parking brakes trigger the brake lights? I also wonder about regenerative braking in electric cars where they slow down simply by lifting your foot off the accelerator.

[aeronaut80]

Lifting your foot off the accelerator in a Nissan Leaf triggers brake lights when in e-Pedal mode (one foot driving, no need to use the brake pedal except in an emergency). It reminds me of a feature that car manufacturers declined to install (due to cost) many years ago, in which brake lights would be triggered by rapidly taking your foot off the accelerator. The idea was that this scenario would usually be followed by emergency braking and that anyone following would see the brake lights come on a fraction of a second earlier.