This is an unnecessarily high jump response in a new suit. It promises adequate support for partner, the values to raise to at least game, and a shortage in the suit actually bid. On this deal West bid 4H over 1S. A 2H response is natural and forcing for one round; 3H is also natural but stronger and forcing to game; so there is no need to jump to 4H to show hearts. Of course this particular Splinter Bid sounds like an attempt to play in the heart game, and is easy to forget but once you have watched partner play in a 3-1 fit you will never forget again.
East knows that he can ruff two of his three small hearts in dummy. Partner promises opening strength for his game-forcing response, and if his points are not in hearts they must be elsewhere.
East asks for controls, discovering that partner has two, probably the ace-king of trump. He can now count six spade tricks, two heart ruffs, ace-king of diamonds and ace of clubs, eleven tricks in all.
Partner will surely have one helpful card in the minors, king of clubs or queen of diamonds – (even if he has overbid with extra heart length there is always the club finesse) – so East bids slam with complete confidence.