The Rotary Salmon Killer - Original non-Echip model is famous for catching salmon.  It is sized to hold a 4 to 4.5 inch anchovy, herring or other baitfish.  The diagonal fin on the back give the bait an erratic rolling action as it is trolled.  The spacer tube holds the hook at the tail of the baitfish where you want it to avoid short strikes.  To load it, you spring the front open and slide the head of the baitfish in until the pins hold it in place.  The photo shows three of the most popular colors which are red, green and chartreuse.