HDPE or teflon is likely what the white stuff is. Some guys like a low friction material that lets you slide the blade left or right while pushing it into the belt. This is the way that Tim Hancock more/less does it, who I believe had a large influence on Travis.
Personally, I use anything from wood, to micarta, HDPE, and even steel, depending on what I'm doing. If I'm pushing something like thin steel through my bandsaw, for instance, I wan't something that's going to grip the steel, like the corner of a hard type of push stick. If I'm grinding similar to Tim Hancock, I like something that the blade will slip against, but still keep a moderate pressure.