in a nutshell, that which is accessed most often will be placed on the SDD portion for fast access, that which is just occasionally read out or in "long term storage" is written to the HD portion.
initially, all data is written to the SSD, after a while the data is sorted as the SSD becomes full and file access frequency becomes more concrete.
in the end, generally the files needed for boot and and files accessed all the time (like a swap file) will be on the SSD.
the issue becomes, if you where to use this in a machine that is on all the time or a game system you'd boot once and that's it. So all the constantly accessed game data ends up on the SSD and your system boot would end up on the platters so it would boot slow... but your game file access would be good.
a hybrid drive is good in a few situations, but far from ideal for others.