Well, I'm still not the expert, but I believe the footed shafts are built so that each finished shaft measures out properly when placed on a spine tester, sanding slightly if need be, and thus taking on the same dynamic spine in flight as any unfooted shaft built to the same specifications, and that's made possible due to spine being purely a function of properties extrinsic to the type of wood itself and, rather, intrinsic instead to an idealized structural cylinder of particular length: such as modulus of elasticity and length.