Many microwave ovens have a mode stirrer, a free-wheeling metal fan at the exit of the waveguide. The ventilation air flow makes it spin, and the angled vanes

Many microwave ovens have a mode stirrer, a free-wheeling metal fan at the exit of the waveguide. The ventilation air flow makes it spin, and the angled vanes reflect the microwaves more or less evenly throughout the oven.