The handkerchief is the dramatic device that causes Desdemona to be seen as guilty and untruthful in Othello's mind.
The moor's reaction to the handkerchief falling into a trance is a good dramatic device both visually on stage and in displaying the contrast between Othello at the start of the play and Othello now.
Its sudden onset would undoubtedly shock the audience.
The General's incoherent would also be disturbing, indicating his change in character to the audience to someone who is jealous, insecure, unconfident, unsure and manipulated.
Iago decides to exploit the relationship between Othello and Desdemona.
He does this by manipulating Othello into believing that Desdemona is having an affair with Cassio.
Othello's short sharp questions convey his increasing doubt and confusion regarding Desdemona. By act 4 scene 1, Othello's character has undergone a complete transformation. Shakespeare shows this to his audience in his language.
He is rambling and anxious and looses his dignity as he falls into a trance. He repeats himself "Lie on her, lie on her and lie on her" "Confess, confess, and confess" "Handkerchief, handkerchief and handkerchief" Overall in this scene the language used by Othello gives the effect of escalating violence, increasingly aggressive and jealous thoughts conveyed through images of poison, decay, and jealousy.
Othello makes his murderous intentions known "Get me some poison Iago" This is a metaphor, jealousy is a poison.
Othello says of Desdemona "Let her rot, perish" Regarding his jealousy "As if there is some monster in his thoughts too hideous to be shown" This has the effect of making the audience think of danger, evil and violence. Conclusion It is the visual, physical proof of Desdemona's infidelity for Othello, but obviously not to the audience who know the truth.