Imagine that your trunk (and the muscles in it) are a cylinder, like a soda can. The pelvic floor muscles are the bottom, the abdominals and lower back muscles are the sides of the can, and the diaphragm is the top (though we could take it further and talk about how the vocal cords may actually be the top! But maybe in another post). With a soda can, if it has not been opened, then there is a certain amount of pressure inside the can that makes that can very strong. One could stand on it and likely not crush it. Now, open the can, or keep it closed and put a dent in the side. If one tries to stand on it now, it will be easier to crush. This is partially due to the pressure in the can being changed.