As stated above, yes, it uses the turbo as an exhaust brake
. It's the same thing Ford GM and Dodge offer on the new trucks. The only difference is the newer trucks have additional controls built in to make the feature work more specifically in tow/haul situations.
Basically, it commands the turbo to increase exhaust back pressure above idle and off throttle. It can be changed to be extremely aggressive or progressive. I have worked on these settings in my tuning for some time now, and I have them set up so that I can let off throttle on the flat and coast at 1500-1800rpm in 5th towing, but when I want to decend a grade, drop to 4th or 3rd depending on the grade and hold my speed to 45-55.
I can tell you that if you use an aggressive street, performance or race tune with the exhaust braking enabled, you will get turbo surge as rpm drops. Also, SCT sets the feature a bit too aggressively in their preloaded devices. You will do better with custom tow tunes with it enabled.
The Current Truck (Oct-2013 to Present):
1999 (Early) F-350 Lariat, CCLB, SRW, Black/Tan
7.3L Powerstroke, 4R100, 4x4, 3.73, 37x13.50-18s, '05 Axle Swap, 6" Fabtech lift, GTP38 "D66" turbo upgrade, SCT 4-position chip with custom tuning by me, Straight Piped 4" down pipe through 5" dual stacks, N-Fab full length triple steps, Pro-Tech cab guard and tool box, louvered 5th wheel gate, Hella 500 Fog and Driving lamps, much more to come...
The one I'll miss, (Mar-2009 to Jan-2014):
2000 Excursion XLT 4x4
6.8L V-10, 4R100, 4.30, 285/75-18, 271whp / 388wtq
The one I won't, (Jul-2005 to Sep-2012):
2005 F-350 XLT, CCLB, SRW, 4x4 FX4
6.0L PSD, ZF-6S650, 3.73, 35x12.5-18, 4.5" Lift