I saw someone on youtube do this and thought it gave a cool look, so I installed it on my Leaf:
It's the DIY Tail-as-Turn module from diodedynamics.com. I needed to add load resistors too. I rewired the stock rear turn signals as additional reverse lighting.
The modules seem to be simple XOR circuits, so when you have the turn signal on and press or let off the brake it will invert the flashing. That inversion blip looks a little odd... I assume cars that have combined tails stock do this more cleanly. But for $30 I think it's a neat mod if you don't mind snipping some wires.