few days before your permit is due to expire you apply for work permit extension http://www.cic.gc.ca/english/information/applications/extend-worker.asp
on the basis of pending PNP nomination,which isnt a valid ground anyway, but it will buy you time. If you send it by paper the processing time is around 3 months, in which you are in implied status and cant leave Canada.
Once you receive the nomination, you send it to the same office. This is what I was told by a lawyer, when I asked the same question.
Good luck. Hope you get it soon.