Movement within your decoy setup is the key to successful decoying on snow.
In wintry conditions pigeons will keep unnecessary flying to a minimum and they also tend to keep in flocks which makes the job of locating other pigeons which have found food much easier.
It is also why you get that ?follow my leader? type of flightline, where the birds stream out of a wood or belt of trees in a continuous flow.
Obviously, what these pigeons see is a constant fluttering and general activity at the business end of the flightline, and this is what you, as a decoyer, should be trying to replicate.
In situations like this a rotary machine comes into its own as do flappers and other decoys that create movement as they bob and flutter.