The external pot on the breakout board will only have effect if the PSU is from a revision that can be adjusted
I personally tested revisions from 6 to 10, and 6 to 8 are working. Can't remember if the S9 worked too.
Clockwise will give more offset from the stock value of 12.2v
Counterclockwise will reduce offset to a point you should reach close to stock value.
Setting the external pot in middle position without touching the internal ones should give you 11.8v undervolt and 12.8v overvolt
I don't think S9 works. At least not according to this post
http://www.rcgroups.com/forums/showpost.php?p=19881042&postcount=997 you need to reconnect the pins for the voltage mod.