[racket] how does call/cc and let/ec exactly works?

hello
I am thinking of such a code like..
#lang scheme
(require racket/gui)
(let ([v (let/ec out
               (dynamic-wind
                (lambda () (display "in "))
                (lambda ()
                  (display "pre ")
                  (display (call/cc out))
                  #f)
                (lambda () (display "out "))))])
      (when v (v "post ")))

but I still have no idea how
"in pre out in post out " is printed but not "in pre out in pre post out "
how does call/cc and let/ec exactly works?
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