When leader knows, he tells partner

Henry Morgan, a 17th-century Welsh pirate who along the Spanish Main became a multimillionaire in today's dollars, said, "A kleptomaniac is a person who helps himself because he can't help himself."

In yesterday's deal, East had to help West steal the contract from South. In today's deal, West can return the favor - how?

South is in three no-trump. West leads a fourth-highest spade six, which is covered by the three, 10 and jack. South immediately attacks diamonds. What should happen after that?