Hmm, my guess, working only from that description, would be that they've created a RAID-1 mirror with equal-sized SSD and spinning HD partitions... but are able

Hmm, my guess, working only from that description, would be that they've created a RAID-1 mirror with equal-sized SSD and spinning HD partitions... but are able to route reads based on the actual position of the spinning disk heads. Then, 'random' reads will usually come from the SSD, but those that happen to be under-the-head (as most often in long sequential reads) would come from the HD. I don't know if usual RAID software/controllers are already optimized for such wildly-different device response speeds (as opposed to uniform devices).