Explain how Iago, being of lower position than Othello, tries to over power him through controlling Cassio?

Iago obtains power by creating emotional turmoil in his commander. Iago's position becomes inconsequential when he can manipulate Othello into a position where he takes control. The success of Iago's mind games afford him a source of power that Othello did not possess. Note, Cassio is nothing more than a tool in Iago's plan.