These servers run the VPS images on a RAID array of SSDs.
The SSDs increase the disk IO throughput and decrease disk IO latency (particularly on 'random access' activities, like database inserts/queries).
You can order a new VPS (just select the SSD option on the ordering page). Or if you have an existing VPS you can move it to one of the SSD backed hosts. e.g. select host895 on the move page. Currently the SSD-backed host option is available just in Dallas. If you need it in other data centers just let us know (here in the comments or via email).