couple things i've recently discovered about the manual button:

in D, ithas exactly no effect. romping the gas below 30mph will still put you in first gear and power mode..

in 3 it will keep you out of 1st, but i think it will keep you out of 2nd as well.. or at least it seems that way

and in 2 it will keep you our of 1st..

it will also keep you out of power mode in 3, 2, and 1.. however if you enter power mode and THEN press the manual button in 3 2 or 1, it will stay in power mode until you let off the gas and then operate in manual mode..

and yeah, first gear is WAY too hot for the snow.. second gear is truly where it's at. but be careful downshifting when you're at speed.. it can start the car bucking around if you're on snow when you do it..

-adam