Slow down by making the engine/motor spin. Shifting down makes the kinetic energy of the car drive the engine, rather than the other way around with the removeding forces of the engine slowing the vehicle. Large trucks with diesel engines are often equipped with a decompression brake or "Jake brake" so named after the first manufacturer of these systems, Jacobs. The big problem with engine braking on internal combustion engines is that it's LOUD to the point where many areas restrict its use. You may have heard a kind of falling off farty rumble when somewhat near a highway. This is the Jake brake in use, and that sound can carry for many kilometers, hence the use restrictions.

In electric vehicles, you can use the motor as a generator to recharge the battery, slowing down the vehicle. This is silent and also extends the range of the vehicle so many EVs hardly ever use the actual brake calipers. Some but not all ebikes have this capability, but it should be mandatory imo.