This deal is from the Tollemache qualifying event.
Once North had opened South knew that slam was a lively possibility.
He forced to game with a two heart response and then showed five-five in hearts and clubs and slam suitability with his four club bid.
North gave preference to hearts with his singleton king. South now checked for key cards and after finding that two were missing settled for five hearts.
Burn's first law says that you should have more trumps than your opponents. This deal is a sort of exception, when trumps broke four-three there was no problem in the play. Three notrump would have been a safer contract, but if North's spades had been Axxxx instead of KJxxx then six hearts would have been a fine contract.
(Burn's second law says that you can't make three notrump on a cross ruff).