Here is nice video I found on YouTube explaining the whats and the whys of Switch rod. There are a lot of confusion out there as to what is a Switch rod and to make matters worst, people are are often mixing it up with the switch cast (a technique in Spey casting).
Lets get it out of the way. - the Switch rod has nothing to do with the switch cast. You can "switch cast" with any fly rod whether it is single hand or double hand rod. Switch cast is just a Spey casting technique. A switch cast is essentially a roll cast done with a dynamic D-loop. You can refer to my earlier post ( http://www.yuenmah.blogspot.com/search/label/Switch%20cast ) to understand the difference between the roll cast and the switch cast. The switch cast is also sometimes referred to as the jump roll.
Now back to the Switch rod; the Switch rod is essentially a double hand rod that is between 10 ft to 11+ ft long. You may think of it as a baby Spey rod. The concept of Switch rod is relatively new in the Spey casting scene and evolved out of the need to fish both single handed and double handed at the same time. The video will explain in better detail. Enjoy!