When shaking the bags, or rolling the cans, have towels or paper towels available for clean up of drips or spills. When ice starts to melt, it is 32 degrees. This is not cold enough to freeze ice cream.
Heat flows from the ice cream ingredients to the melting ice.
The melting ice warms up while the ice cream becomes cold enough to freeze. Shaking the bag does two things. It brings warmer ingredients in the middle into contact with the cold outside and speeds up the freezing process. It also makes the liquid ingredients mix with air. Zip bag and place inside the larger one.