When the cap is removed, the pressure is suddenly released. The water is now above the boiling point and bubbles form throughout the water very quickly, leading to the explosive boiling. The wood will melt more ice because it will need to lose more energy to cool down to 0oC because wood has a higher heat capacity. The water covers the fruit. Before the fruit can freeze the water has to drop to freezing.
Water has a large heat capacity so it gives off a lot of heat as it cools to freezing.
This heat warms the fruit and the air near the fruit. This trick will work for temperatures just barely below freezing. It won t provide enough protection if it is much colder. This problem has to be done in stages. For this step the heat capacity of ice is used.