When the sap falls ?????

All my life I have heard "when the sap falls" in connection with deciduous trees and autumn. I have two questions on this, based on the veracity of the first statement.
One:
Is this what actually happens? Does the "sap" go down into the roots?
If the above is true, when the leaves have turned, but not necessarily fallen yet, has the sap already "fallen"? Is this a good time to prune limbs?
Two:
I have some limbs that hang downward to get into the sun (maple mostly, hanging down from the point of their attachment to the trunk, how does the sap "fall" up?
FACE <yes, I am serious.>
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No. Please see: http://www.colostate.edu/Depts/CoopExt/4DMG/Garden/Amazing/sap.htm sed5555
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