How to bake chips in microwave using grill?
According to Consumer Reports, these microwaves typically have traditional heating elements in addition to microwave heat, or they're combo convection/microwave ovens. The rack is to move food closer to the grill element, or to allow better airflow for convection models.

In that case, you'd use the microwave like a small grill or convection oven rather than a traditional microwave - see the instruction guide for your make and model (typically available on the manufacturer's website.) In this way, you can prepare crispy items like oven-baked tortilla chips or samosas (I prefer oven-baked samosas over fried myself! Mmm.)