Plunking when it refers to salmon steelhead it usually refers so throwing a bait/lure out in a high fish traffic area and waiting for a bite. I doubt many people actually plunk with shrimp alone, they more than likely have some drift bobber also to keep the bait up off the bottom. Spin-glo's for instance are very popular for plunking and in a large enough size can easily float a shrimp a few feet off the bottom. I think the key to being successful it to be in as I said a high traffic area. That is an area where and when a lot of fish are moving through. beloe bonneville dam during the spring chinook run for example.