The Legcap is designed to keep your bottles of soda from losing their fizz before you finish drinking them.
Everyone knows that carbonated drinks start going downhill immediately after you open them, at least in terms of fizziness. Putting the cap back on helps, but it's not a perfect solution: even if you screw it on as tightly as you can, you won't have a perfect seal. Carbonation will still move upward and out of the bottle. Jaeseok Han came up with a solution that ensures your bottles don't lose their carbonation: the Legcap. Rather than using the cap that comes with your bottles, cover them with the Legcap and turn them over. The design of the Legcap will seal the bottle so your favorite drink won't escape, as well as balance the bottle so it won't fall over. It will still sit comfortably in the fridge and the bottle cap is standardized to work with most water and soda bottles.
The Legcap doesn't solve all the problems associated with keeping carbonation in a bottle--pressure is still lost when you open the bottle the first time--but it certainly helps with the problem if you aren't going to be able to drink the whole bottle in one sitting. It also makes bottles easier to open, at least after that first time you take the top off.