This is because the reaction cross section is substantially increased. To increase the impact of the tamper on the fissionable shell, you can levitate the shell. You leave an air gap between the two materials. This has the effect of increasing momentum transfer, and hence facilitating implosion. Comprised of 2 parts.
Larger mass is spherical and concave.
Upon detonation of conventional explosive, the smaller mass is violently injected and welded to the larger mass. Supercritical mass is reached, chain reaction follows in one millionth of a second. These 32 sections together form a sphere. The shape of the detonator resembles a soccerball.