The only snap-back mechanism--and that's usually what's referred to as a snap-back to previous tariff levels--is the only one I think we as a group are familiar with. It's the bilateral safeguard chapter, where if there is injury caused by the elimination of tariffs over time, there are provisions during the elimination of the tariff period and sometimes a year or two beyond to take safeguard measures.
On April 28th, 2008. See this statement in context.