The outcome of decades of two-dimensional modelling of lithosphere deformation under extension is that mechanical coupling between the continental crust and the underlying mantle controls how a continent breaks apart to form a new ocean. However, geological observations unequivocally show that continental break-up propagates in the third dimension at rates that do not scale with the rate of opening. Here, we perform three-dimensional numerical simulations and compare them with observations from the South China Sea to show that tectonic loading in the direction of propagation exerts a first-order control on these propagation rates. The simulations show that, in the absence of compression in that direction, continental break-up propagates fast, forming narrow continental margins independently of the coupling. When compression is applied, propagation stagnates, forming V-shaped oceanic basins and wide margins. Changes in out-of-plane loading therefore explain the alternation of fast propagation and relative stagnation. These new dynamic constraints suggest that the west-to-east topographic gradient across the Indochinese Peninsula prevented continental break-up propagation through the 1,000-km-wide continental rift of the central and west basin of the South China Sea, until the direction of stretching changed 23 million years ago, resulting in bypassing and acceleration of continental break-up propagation.