When the primary explodes, the X-rays escaping from the fission trigger fill the radiation channel, the space between the bomb casing and the fusion capsule, with a photon gas. This space is filled with plastic foam, essentially just carbon and hydrogen, which becomes completely ionized and transparent as the x-rays penetrate. The inner casing and outer capsule surfaces are heated to very high temperatures. Thermal equilibrium is established extremely rapidly, so that the temperature and energy density is uniform throughout the radiation channel. This ablation process, essentially a rocket turned inside out, generates tremendous pressure on the fuel capsule and causes an accelerating implosion.
Thermal equilibrium assures that the implosion pressure is very uniformly distributed.
The force that compresses and accelerates the fusion fuel inward is provided solely by the ablation pressure. It is unlikely that the fissionable rod reaches such extreme compression however. Located at the center, it will experience an extremely violent shock wave that will heat it to high temperatures but compress it only modestly, increasing its density by a factor of 4 or so. This is sufficient to make the rod super-critical. Depending on the degree of symmetry, and the physics of the particular capsule collapse process higher densities are possible.