USCIS has “wait-listed” some H-1B petitions, meaning they may possibly replace petitions chosen to receive an FY-2009 cap number, but that subsequently are denied, withdrawn, or otherwise found ineligible.  USCIS will retain these petitions until a decision is made whether they will replace a previously selected petition.  USCIS will send a letter to the wait list petitioners to inform them of their status

Can someone clarify this?? It will probably take some days for uscis to send a letter to the wait list petitioners and send the documents back to uscis. And also if these petition can replace previously selected petition, how can uscis complete the premium processing in 15 days??