manu mode will lock the car in the selected gear... it will not shift for you automatically like tiptronic though (so beware)
it does not yield faster acceleration... it does not yield any potential benefits...
if you have your trac fused removed; it will not work
the only use i have ever had for it was for spooling on the roll...
ie if you are on the highway, and want to brake boost, lock it in second (do not drop it into second) then you can take off spooled, rather then waiting for a downshift