How does letf work?

[Florian Beck]

The docstring of letf says: "This is the analogue of `let', but with 
generalized variables (in the sense of `setf') for the PLACEs."

However, with setf, I get this:

(defvar test-x '(KEY 1 2 3 4))

(let ((x (copy-list test-x)))
  (setf (cdr x) '(a b c d))
  x)

=> (KEY a b c d)

(Without copy-list, the global variable is modified.)

Yet this doesn't work with letf:

(letf (((cdr test-x) '(a b c d)))
  test-x)

= > (KEY 1 2 3 4)

So, either "generalized variables" in the sense of setf are different 
from the sense of letf or -- more likely -- I don't understand how they 
work. Any hints?

--
Florian