The Plan does not hold itself. A hidden hand watches the trajectory drift from Seldon's path and applies the smallest nudge to return it — the Second Foundation, correcting from the shadows. Switch it off and the future wanders; switch it on and it tracks the plan.
The Seldon path is a fixed target curve; the actual trajectory takes a random step each tick (drift) plus, when the corrector is on, a restoring nudge = −k·(actual − target). This is proportional feedback control: with the corrector, the error stays bounded and the trajectory tracks the plan; without it, the error random-walks away (variance grows without limit). Toggle it and the divergence is immediate and measurable.
'The Second Foundation correcting from the shadows' is Asimov's device. What runs is a proportional controller pulling a noisy signal back to a reference — the plan holds only because something actively corrects it, which is the honest content of the metaphor.