Fan-type Expander is used in cases when anterior section needs to be expanded more than posterior. The screw could also be used in reverse to expand the posterior more than anterior.
When a case presents adequate molar width but could use just a bit of anterior expansion the Fan Expander can do the job. An expansion screw in the anterior pivots on the posterior hinge as it is opened. With proper acrylic relief during expansion you will be able to see amazing results.