This is the #1805 Rotary Salmon Killer Bait Holder with the Blue Plastic head.  It is rigged with a seven foot leader and a 4/0 Siwash hook.  The Rotary Salmon Killer is the original and most famous Pro-Troll bait holder.  It has caught hundreds of thousands of salmon up to 50 pounds and bigger.  It is spring loaded to firmly hold your bait in place.  On the back is a diagonal fin that creates the perfect rotation of your bait.  The spacer behind the head holds the hook at the tail of your bait where you want it.  This rig is a sure bet for catching salmon.    
This baitholder is best for 4 to 5 inch long bait.  Other Big Fin models work better for bigger bait.  To load the Rotary, pry open the plastic jaws with your fingers.  Insert the baitfish in just far enough to catch the teeth in the head of the bait.  As you put it into the water, watch to see that it is rolling.  Sometimes the bait is curved and it does not want to roll.  If so, turn the bait over or reposition it in the holder.  You can cock the bait slightly in the holder to slow down or speed up the roll.