actually locks out 1st and 2nd gears, always starting the car out in 3rd gear. The Transmission Control Module (TCM) bases its decisions on inputs from sensors in the transmission as well as information supplied to it via the Closed Area Network (CAN), from the Engine Control Module (ECM). Among those decisions are the transmission's actual shift points. Primarily, the TCM bases shift-point decisions on the accelerator pedal position, vehicle speed, and engine speed. As far as I know, Winter Mode
doesn't really alter shift patterns after the intial 3rd gear start. The more complex tasks are taken on by STC and DSTC.
I hope this helps!