Gizmag gives us the details:
"To make the topping, regular Ichiban beer is frozen to -5 degrees Celsius (23 degrees Fahrenheit) while air is continuously blown into it. It's kind of like when a child makes bubbles in their drink, except inside a blast freezer. Once the topping is placed onto regular, unfrozen beer though, it acts as an insulating lid and keeps the drink cold for 30 minutes.""
Link to Original Source