The external field effect in the substrate framework. The parent galaxy's ebbing current imposes a DC phase bias on counter-rotating boundaries within its gravitational influence (purple gradient). At each TDG, two arced arrows represent the counter-rotating substrate flows whose parity symmetry produces the MOND gravitational response (GD2–GD4). Near the parent (TDG A): the external field is strong, the bias breaks the parity — one flow dominates (thick arrow), suppressing the MOND enhancement. Far from the parent (TDG C): the external field is weak, the two flows remain symmetric, and the full MOND response is recovered. The teal bars below show the resulting MOND enhancement gradient. This effect is calculable from the parent's baryonic mass and the TDG's orbital distance ($g_\text{ext} \approx \sqrt{a_0 G M_\text{parent}}/D_p$), with zero free parameters — and directly testable by comparing rotation curves of TDGs at different projected distances from the same parent system. CDM predicts no such distance-dependent effect.