I’m about to explain the ‘principle of uniform pressure’. But first, I need to talk about something else first. The centre of buoyancy (CoB) is the average mass of an object. Whereas the centre of gravity (CoG) is the average effect gravity has on the object. So if I were to attach a bunch of styrofoam to my chest, my CoG would change very little but my CoB would change considerably more. The opposite would ring true if I replaced the styrofoam with a lead weight. Why is this relevant? Well, your CoG is the thing your internal circuitry is trying to keep under control. Most of the time in martial arts we talk about a person centre and we really mean the CoB. Controlling the CoB can allow us to apply a torque to the CoG.— references

[1] Center of buoyancy, Wikipedia http://en.wikipedia.org/wiki/Buoyancy
[2] Center of mass, Wikipedia http://en.wikipedia.org/wiki/Center_of_mass
[3] Image credit http://vignette1.wikia.nocookie.net/schools/images/e/ef/Scn0003-1.gif/revision/latest?cb=20060611194343 If you wanted to read a bit more about these ideas, you can check out the concept of metacentric height http://en.wikipedia.org/wiki/Metacentric_height#Metacentre