The "ECT" stands for Electronically Controlled Transmission. The "Snow" mode starts the transmission in 2nd gear rather than first so that the wheels don't have quite as much torque and are less likely to spin in inclement weather. Secondly, it changes the whole shift pattern of the transmission, particularly when accelerating from a start or when passing. It has the transmission change into higher gears earlier so that, again, when it is bad conditions you are less likely to spin the tires each time the transmission shifts up and when you go into a passing gear, it down-shifts only one rather than the usual 2 gears so that you don't have as dramatic a change in the power going to the wheels.
Sourced from a 15 year vetran of wrenching on Toyota's
'06 Toyota Highlander 3.3L AWD - Chrome Grille, K&N Air Filter, Tinted Windows, Vent Visors, PIAA Bulbs, Mobil Fluids
'87 Honda CBR 600F Hurricane - Modded