OK! That is how I would describe the fit and function of the rafter assembly fixture. Looking at the pictures you get the idea that there are several pieces that must first fit together. Then realize that when assembled they must fit over the top truss chords and inside to the cleat and seat that will brace the roof to the truss.
Above - This is how the rafters will be installed and mated to the Lateral Braces. The Primary Rafters are removed and added later.
Puzzler - Why do the Primary Rafters need to be removed and installed later?
Tech Vocab - Lateral Brace - Racking - Mortise - Knee Brace