The smart stop doesn't work on the stock Slash esc - it works on orig EVX in the 3906 or the EVX2 in the 3905 just fine.
The smart stop is wired to the battery that runs BEC, rx and servos - this batt is used more and will go low voltage 1st. Protect that batt and the other is automatically protected. There is no place on the outside of the ESC where you can get 16.8v (or 14.4v) and use a 4S smart stop, reason you use a 2S - unless you want to dig into the EVX.
If you plug one batt into the EVX and then turn switch on - if esc light comes on, thats the side that runs the bec, rx and servo - this is the side to wire the smart stop to. Note motor will not run this way. If esc light doesn't come on, thats not the side you want.
Smart stop is the best option to use on a 3905.....I'm doing it to my 3906, just haven't finished since weather here is rain.....