I do not understand input-decode-map

[Lennart Borgman (gmail)]

This works, ie typing "." gives ":" (even in for example Viper)

  (define-key input-decode-map [?\.] [?\:])

Why does not the next work?

  (setq input-decode-map (make-sparse-keymap))
  (defvar my-second-map (make-sparse-keymap))
  (define-key my-second-map [?\:] [?\.])
  (define-key input-decode-map [?\,] my-second-map)

Typing "," does what it use to do. Not too bad, but not what I expected ...