When treating pigmented lesions, such as photodamage, the light is absorbed by the melanin making up the brown spot. The body's natural processes work to get rid of the damaged tissue, revealing more youthful looking skin after about a week. Make sure not to pick at the spots, they will flake off like coffee grounds for about 7-14 days after your treatment. Looks like you are going to have a great result from your IPL treatment. Make sure to protect your skin from the sun while your skin heals.
Fine crusting and darkening of sun spots is normal after IPL. You will probably have a very nice result, with removal of most of your freckles and sun spots. However, It is important at this time to follow strict sun avoidance, use a good moisturizer twice daily and not to pick any of the crusts.