Each control point is linked to one H and one V scaffold (the points linked to H-scaffold 2 are highlighted here). Scaffold lines are shared between multiple characters; H-scaffolds 1 (dark-green) and 2 (numbered) are common to almost the whole font. 2 is a child of 1, so when 1 moves, 2 moves with it to keep the height the same (it prevents, say, the base being rounded down and the top being rounded up, making the P too tall). H-scaffold 3 (red) is a D-tangent positioning the bottom of the P's loop. It is a child of 2.
Similarly V-scaffold 1 (the stem of the P) is shared by the characters B, D, E, F, H, K, P, R, Eth and Thorn. The R-tangent 2, a child of 1 shared by P and R, keeps the width of the loop consistent.
There is a simple hint in the loop to ensure no dropouts.