No You don't !
You actually leave Canadian Border, -------------> Walk through the No Man's Land ------------------> Reach USA border control and tell them your intentions of flag polling-----------> They take your passport, put a denied entry stamp as you don't have visa --------> Show you way back to Canadian Border.....-------------> You Walk through the No Man;s Land again, ------------> Reach Canadian Border, and are Welcomed as a PR.
I hope it helps.
Good Luck !