This puzzle is derived from a real game of bridge I played on Bridge Base Online.
I was sitting North on board 9 (N dealer, E-W vulnerable) and the bidding turned out like this:
East as declarer won the first seven tricks and reached this position with the lead in his hand (obviously he would not know about NS's exact holdings in-game):
♠ AT ♡ K643 ♢ - ♣ - ♠ J9 N ♡ Q7 ♠ K ♢ AJ W E ♡ AT8 ♣ - ♢ 6 S ♣ 7 ♠ Q85 ♡ J95 ♢ - ♣ -
He ended up down one and my side bagged 400 points. However, given that I last played the ♠4, there was a justifiable line of play that would allow him to make the contract against best defence.
How can East make his redoubled 6♣?
There is no one set of exact card plays that works for all situations. What the defenders play has to be taken into account!