I happened upon the manufacturer's site for a company named Backer Rod who makes backer rod products . Backer rod is apparently a trademarked name and a common name (Kleenex/kleenex) for the extruded foam cord you stuff down into your coping/deck expansion joint before you apply sealant.
The pdf catalog linked on their home page, http://www.bayindustries.com/backer-rod, contains a terrific technical article that explains all the details of selecting and installing a foam filler cord and it turns out there's more aspects of filler cords than you ever thought possible. It's a must read for anyone doing their own sealing because their recommendations will help ensure the sealant you apply on top has the best chance of flexing without breaking throughout its useful life.
Some of the horror stories I read of pool deck joint sealants not adhering or splitting or bubbling or lifting out may be have been avoided with proper filler cord selection.