It isn't really a short. It is covered by restricted stock and is done on contract. The seller gets his money less prevailing interest rates when he delivers the cert with the legend removed.
The Frankfurt (or Berlin) is often used by less than reputable penny stock companies (scammers) to sell restricted stock and circumvent the rules.
We are apt to shut our eyes against a painful truth... For my part, I am willing to know the whole truth; to know the worst; and to provide for it. --Patrick Henry, Patriot and Hero of the American Revolution