window-resizable confusion

[Chong Yidong]

  (window-resizable WINDOW DELTA &optional HORIZONTAL IGNORE TRAIL NOUP
  NODOWN)

  Return DELTA if WINDOW can be resized vertically by DELTA lines.
  ...
  Optional argument NODOWN non-nil means don't check whether WINDOW
  and its child windows can be resized.

I don't understand the NODOWN argument.  When NODOWN is non-nil, the
docstring says the function doesn't check whether WINDOW can be resized.
So what else is it doing instead?