Skimming and scanning are two very different strategies for speed reading.
They are each used for different purposes, and they are not meant to be used all the time. They are at the fast end of the speed reading range, while studying is at the slow end.
People who know how to skim and scan are flexible readers. They read according to their purpose and get the information they need quickly without wasting time. They do not read everything which is what increases their reading speed. Their skill lies in knowing what specific information to read and which method to use.