It pushes against the outside pressure, it doesn t allow any flow though it. It seems sometimes that a capacitor is allowing current to flow through it because you see the current come off the back side. But nothing is going through the capacitor. In the case of a balloon, you are displacing air as the balloon inflates and if it was inside of a can or something you would notice it, but since it is in the atmosphere you don t notice. Sometimes air can flow from one side to the other.
Real devices have real properties.
Balloons do so catastrophically as do most electrical capacitors. OK, enough of that, you get the idea pneumatics are an excellent analogy of electronics. The flow of electrons is surprisingly similar to the flow of air. This is because they are really both fluids. Air is the fluid of gas molecules slamming into one another trying to spread out.