I could explain it in my bad english, but looking at this page is the fastest way:
http://www.adaptec.com/en-us/_common/hybrid-raid/
Do you know of any linux implementation for this outstanding feature? (to be so able to use it without the adaptec controller?)
There is at least BCache kernel patch for the brave souls; I have no idea if it's maintained, though. Any better suggestions are gladly welcome.
+1 for interesting question!