In Act 4 scene 1 Iago is starting to put his plan in full affect by really getting into Othello’s head about this imaginary love affair that Desdemona is having. He is using, what seems to me, is a form of reverse psychology and he’s speaking in a way that is saying out-right that Desdemona’s actions are harmless while implying the exact opposite. Iago states “or to be naked with her friend in bed an hour or more, not meaning any harm” and then in reference to the handkerchief he gifted her he states “why then ‘tis hers, my lord, and being hers, she may, I think bestow ’t on any man.” It may seem like Iago is defending Desdemona here but in reality he is just trying to infuriate Othello more by making it seem like what she is doing is ok. He is talking about how it is ok for a woman to lie naked with another man as long as they’re doing it as just friends, in which case it’s supposedly harmless. By speaking about it like this he is taking the affair a step further and making Othello imagine the two of them naked in bed together. Now Othello’s fury is escalated because rather than just having the belief that the two slept together he has now created a mental picture. An image, that while not a real image, is burned into his memory escalating his fury. The same thing is happening when Iago is mentioning the handkerchief that Othello gave her that belonged to his mother and supposedly is the reason that his parents stayed together back in the day. He is creating a mental picture of Desdemona giving this beloved heirloom to another man making Othello blind with rage and hatred for Cassio and much more likely to believe whatever other piece of evidence is thrown at him.