yes, in WS'ing, the seeds are sown in flats, earlier than we would put them in the ground. Then the flats are put outside, no matter the weather. The results are plants that are hardy.

Plus, flats for many of us translate into some kind of a covered plastic container. What we're aiming for is to create an inexpensive greenhouse, where the seeds will get an early jump on life in spring. Think milk jugs, 2-liter pop bottles. That kind of thing. Just be sure to puncture holes on the bottom of the jug for drainage and on the top for ventilation.

You loose many seeds to animals and run-off so they need protection as tempusflits stated above. Most winter sowed seeds are the ones that need a cold wet period called stratification. In nature plants drop their seeds and the ones that survive (being buried, animals eating them and run-off) will sprout in the Spring when the weather warms up.

Flats tend to be very shallow for winter sowing. Using milk jugs or 2 liter bottles will give you better results.