West leads the diamond queen against four hearts, and declarer can see four losers: two clubs, a diamond and a spade. The only chance is to set up a spade trick for a diamond discard, and it must be done quickly.
Should South broach spades by advancing the 10, he can legitimately develop an extra trick if West has honor-doubleton or -tripleton in spades. West can either take the first spade, exposing East to a ruffing finesse, or he can let East win it, in which case West’s honor can subsequently be ruffed out. However, declarer can do better by starting with a low spade from dummy. This puts East to the test. If he sleepily plays low, declarer can subsequently run the spade queen to set up the diamond discard.
East should be wary of this tactic and rise with the spade ace. From then on, declarer cannot succeed. He can either take a ruffing spade finesse or try to ruff down the king, but neither works.
This low spade gambit is, of course, best tried at trick two, when little is known about the deal and plenty of entries remain in dummy. Yes, this might run the risk of losing to a club ruff, but we are only talking undertricks if that is the case.
Just as in yesterday’s deal, the defense becomes harder (some would say impossible) if the spade ace and king are switched. However, you could certainly argue from East’s perspective that four hearts cannot be defeated if declarer has the spade ace. East can almost count 10 or more winners for declarer if South does indeed have the spade ace.