Suppose a fixed mass of gas is kept at a constant temperature, and the volume it is held in is increased. The pressure of the gas decreases.
The temperature has not changed, so the molecules still move at the same average speed. But the container is now larger. Each molecule has to travel further before it reaches a wall, so collisions with the walls are less frequent. With fewer collisions each second on every square metre of wall, the average force on the wall is smaller, so the pressure falls. If the volume is made smaller, the molecules hit the walls more often and the pressure rises.
In the marshmallow demonstration, air is pumped out of a jar, so the pressure outside the marshmallow falls. The gas trapped in tiny pockets inside it is pushed back less by the outside air, and the marshmallow swells. When air is let back in, the pressure increases again and it shrinks.
