N/All		♠	7
		♥	7 6
		♦	A Q J 10 2
		♣	A K 7 5 4
		♠	A 5 3 2
		♥	A K Q 8 5
		♦	K 4
		♣	9 3

¹ Fourth suit forcing (game forcing since this is a reverse bid as well)
² Less than three hearts, no spade guard, five clubs (with 5-4 in diamonds-clubs North would made a 'neutral' 3♦ bid)
³ Shows a six card suit...

West leads the ♠K, South winning with the ♠A.
As North's diamond suit turns out to be very good, 7♦ looks like a nice contract (but it is not on a trump lead).
South should forget about 7♦ though, since he is in 6♥. What is the safest way to play that contract?

Solution

Declarer has tricks galore... once the trumps have been drawn.
The spade lead removes South's guard in that suit.
Plan A: declarer draws three rounds of trumps with the ♥AKQ. If the trump suit is 4-2, declarer can do no better than to cash the diamonds. The defender holding four trumps will have to have four diamonds as well, allowing declarer to pitch his last spade loser on the fifth diamond.
Plan B is clearly better: declarer ruffs a spade first, then draws trumps. In that case the player holding four trumps needs to have three diamonds only.
Plan C: declarer ruffs two spades: ♠A, spade ruff, ♦K, second spade ruff, ♦A, ♦Q on which South pitches his last spade. An opponent is welcome to ruff, as long as it is from three or four trumps. Looks good?

		♠	7
		♥	7 6
		♦	A Q J 10 2
		♣	A K 7 5 4
♠	K Q J 9			♠	10 8 6 4
♥	J 10 4 3			♥	9 2
♦	6			♦	9 8 7 5 3
♣	J 10 8 2			♣	Q 6
		♠	A 5 3 2
		♥	A K Q 8 5
		♦	K 4
		♣	9 3

No. All three plans fail.
Plan A: West ruffs the second diamond, the ♦A, and cashes three spades: down three!
Plan B: since South has ruffed a spade he suffers a two trick defeat only...
Plan C: down one only.

On closer inspection: Often Plan C would fail too if the third diamond (the ♦Q) is ruffed, even if the defender holding four trumps does so. He would return a club, locking declarer in dummy. South then has no choice but to play the ♣AK and another club, risking a second trump loser: either by way of a 'promotion' (if West started with two clubs) or by an uppercut if East started with two clubs (he ruffs the third club with the ♥J, the ♥10 or the ♥9).

The best line of play is simple but often overlooked. At the second trick declarer plays a small heart from his hand. This way EW win the heart trick they are entitled to, while dummy still has the ♥7 to take care of the second round of spades. In other words: declarer is in control. He wins any return, draws the remainder of the trumps and claims, since he can pitch the three spade losers on the diamonds (the twelve tricks he scores are: four trumps in hand, the ♠A, the ♣AK and five diamonds).
This line of play always succeeds as long as the trump suit is 4-2 or 3-3.

Note: declarer has no problems on the lead of West's singleton diamond: he simply plays the ♥AKQ and starts on the diamonds.