This is quite simple but if you really understand it, it explains a lot of other things.
We’ve briefly covered the various stages of hypothermia. For regular cold water swimming, the important thing here is that as temperature decreases blood-flow changes. Blood circulation from the extremities to the core decreases, in order to protect the vital organs. The means the blood in the extremities, arms & legs will get colder than the blood in the core. It’s important therefore to remember that you don’t want this cold blood suddenly flowing into your core, as that is the real source of danger.