They are lighter and supposed to reduce binding over long servo-surface distance. You basically replace most of what would usually be metal on the control rod with a wood dowel. Making them is easy, you take a wooden dowel which is roughly 4"-5" shorter than the whole distance between the control surface and the servo. You then glue metal pushrods to the ends of the dowel to complete the needed length. Ive seen it done by wrapping string around the dowel to secure the metal pushrod. About 2" over the wood will do. Then use epoxy to harden the string.

You can always use metal pushrods and there wont be any trouble.