Inside that blue cap sits a small glass ball. The pressure inside the bottle holds the ball against the top of the glass bottle, and in order to get to the soda, you have to pop the glass ball to release the carbonation pressure.
Not my video: How to open a ramune bottle
Simply press the included plunger (red in my photo above) into the bottle to get it open. But here's the trick: you have to press the red plunger down fast enough and hard enough to keep the gas and soda from exploding out like champagne! It's not too difficult, but there's always the chance that you'll mess it up.
The marble sits near the top of the bottle, and there are special indentations in the glass to help you drink it without the marble rolling up and blocking the flow.
And ramune is pretty tasty, too! If I can find more flavors I'll post more info.