When a contract depends on the position of two or three key cards, it helps to make a definite ‘assumption’ about one of them. Declarer then plans the play, ‘assuming’ the key card is badly or well placed, as the case may be.
As South, you open 1♥ in second seat and North raises you to 4♥. West leads the ♣10. East wins with the ace and switches to the ♦4.
With three certain losers (a spade, the ♦A and the ♣A), you will need to guess right in diamonds and pick up the trumps without loss. You intend to finesse in hearts and so assume that East holds the ♥K. This assumption tells you to place West with the ♦A because East, if holding the ♦A, the ♣A-J, the ♥K and possibly a spade honour, would not have passed as dealer.