what does iago witness between cassio and desdemona? what does he think about it?

Cassio and Desdemona walk away from the others to speak about Othello. Iago, of course notices that Cassio touches Desdemona’s hand while he's talking to her, and he decides it to be the perfect thing to frame him with........ Othello won't like it, and he believes that Cassio might just lose his promotion as a result.