...as all hand signals are done with the left hand..
Depends where you are in the world I guess.
Doing all signals with the left hand is great if you're on the left side of a vehicle and only your left arm is visible.
Over here it is is perfectly acceptable (on a bike) to stick your arm straight out on either side to indicate you're going that way. Doesn't work well in a car
When done with one hand, our stop signal is the same as the left turn (obviously we drive on the other side of the road). Your arm pointing down signal doesn't apply here.
Apparently in the US it is also acceptable to indicate a right turn by sticking your right arm out.
However I take the point that leaving yourself with only front brakes, whilst highly effective, can also be highly entertaining for people following you (and not in a good way).
And speaking of riding bikes in the US. This is me on my way to work earlier this year...
From start to end a 60 metre descent -- far less fun on the way back.