This means that I am (at least initially) looking for the best game and that hearts might be a problem. I don't necessarily need a spade support this good, but I must have clubs well stopped if I am planning to pass 3NT (which I am). It would look silly to be anywhere else opposite something like -
With better spades and weaker hearts, partner might suggest 4S, and with no heart stopper, he will probably revert to 4D (which I should be allowed to pass). Over 4D, I intend to bid 4S - suggesting a good doubleton.
Partner will know that my honours are outside hearts, so should be excited with a singleton heart.