The flashmemory in the Fusion Drive moves data and applications frequently accessed by the user to the flashmemory and move less frequently used data to the hard disk drive as the flashmemory becomes full.
These approaches can combine a smaller SSD that stores the operating system and applications and a larger HDD that is used for user data, or a hybrid HDD that has flashmemory inside a HDD that is used to store frequently accessed data on the flash with less used data kept available on the HDD.
Using a combination of HDD and flashmemory in either a hybrid HDD or a dual storage notebook computer, where frequently used data (and possibly the operating system and application software) is kept in the flashmemory, while the bulk of user content is kept on the HDD, has been shown to provide about 80% of the performance improvements of a pure SSD-based computer.