That loses immediately

Why can't black play exf6?

The mate is that there is nothing to do about QxR. 

i don't get it were's the mate?

This is a unique puzzle which is found in The Chess Competitor's Handbook by Bozidar M. Kazic. It shows an example of the FIDE interpretation of the rule on 3 fold repetition where after the repetition. the person on move no longer has the same possibilities in piece movement and because of this fact a request for a draw would be denied. It's on pages 149 - 150. 

great game, nifty puzzle