Step 1, Liquid flux is deposited onto the center of the wafer
Step 2, A low rpm initial spin pulls flux from the center across the wafer surface
Step 3, A high rpm spin ejects excess flux - leaving a thin layer on the wafer
Spin coating is an amazingly simple way to apply flux to a (generally round) substrate. Most commonly used in wafer processing, spin coating can be used as a more uniform alternative to spraying liquid fluxes onto wafers for electroplated bump formation. Here is an image tutorial to introduce the process: