South opens 1♠ and North will bid one of 2NT (Jacoby - showing game values and 4-card support but usually denying an outside singleton), 4♦ (Splinter - game values with slam interets and showing a singleton), 4♠ (Game Raise), or 4NT (asking) depending on style.

East's 5♦ is a brave bid but does it make sense? if North-South don't have the Slam on it will be doubled and looks like it goes 3 off which at -800 is too great a scarifice against a vulnerable game. If N-S do have the slam on it is an ineffective deterrent. In fact it's dangerous as well as ineffective.

If North shows slam interest with either 4♦ or 4NT then 5♦ tells South the North has precious few points in Diamonds. That clue when added to the 'Slam Interest' clue is enough for South to deduce that North has the ♥ A and therefore South should bid 6♠ .