For example you need to write the Unicode-Character for an EM DASH (in Unicode-block 20 character 14).
So first change the input-method in Emacs to rfc1345 with C-x RET C-
Now the input-method for this buffer is rfc1345 and with & and the char-code we can enter every unicode character (for example &-M for the EM DASH).
All charcodes are listet in rfc1345 (i.e. http://www.faqs.org/rfcs/rfc1345.html)
With C-\ you can toggle back to the "normal" input-method.