How does Iachimo convince Poshumus he's slept with Imogen?

Iachimo first gives a general, and then a more specific description of Imogen's bedroom, which Posthumus says he might have learned from anyone, even himself, in conversation, and that even if he saw the ceiling, this is no material proof.

Next Iachimo brandishes the bracelet that he took from Imogen's wrist, but Posthumus counters that Imogen may have given it to Iachimo to give to him. Iachimo says that he wasn't aware that she had written Posthumus that she was going to do so. With that, Posthumus loses his faith in Imogen, takes off the ring she gave him and curses her (and all women's) infidelity.

Posthumus's loyal friend Philario rushes to defend Imogen, and manages to sway Posthumus momentarily, who says he needs a more "corporal" sign. And then, Iachimo reveals that he saw the mole under Imogen's breast.

That does it, and Postumus is ready to make mincemeat of his wife.