All fishing rod blanks today are manufactured by wrapping the rod material around a steel mandrel. This will produce a hollow tube. It is near impossible to produce a perfect tube as the overlaps and joins in the material create unevenness in the wall of the blank. This results in a soft side and stiff side commonly referred to as the spine of the blank. Think of the spine like your backbone. You have a lot more strength bending straight forward than you do to the side.
All fishing rods have a spine, some can even have more than one. There should always be one that is more defined than the others. Some store bought rods are not built on spine and by building on spine we are building a better quality better performing rod. A rod that is built on spine will have the most lifting power, will last longer and cast further.
On a two piece or multi piece rod you must locate the spine on each piece.
We mark the inside of the curve (opposite of the spine) because we are building a spinning rod. All the guides and reel seat will align with the inside of the curve therefore it makes sense to mark there.
The fishing rod can hold a lot of tension when it is bent. Wear eye protection. Be aware of your surroundings and those standing near you in case you lose control of the rod
It is always good practice to repeat the process a few times to ensure that you have found the most pronounced spine.
If you are building a casting rod (overhead reel) the rod guides will go on the opposite side i.e. the outside of the curve or on the spine.