I think what you are referring to are sprockets that just 'suspend' between the upper and lower chains, some refer to them as "majic sprockets". The sprocket is sized to be a slightly higher tooth count (diameter) than the sprockets on the shafts.
We have used them and in some places they work well. They float between the upper and lower chains, and will move back and forth a bit while the chain is running. I think they work best when you can use a small diameter sprocket for the tensioning (2 to 2-1/2 inches) and shorter runs of chain (12" shaft to shaft).
We used them with about a 4" diameter and in a 24" span, and due to the flexibility of that length of chain and the diameters, they would occasionally pop loose if we were hit hard.
AndyMark sells some very simple tentioners that also work well. They were actually designed by a student on an FRC team (from California I think). Here is a link to the product page.
http://www.andymark.com/ProductDetai...Code=am%2D0286