A spherical capsule filled with hydrogen fuel is bombarded with energy, compressing and heating a small region in the center of the fuel. The fusion reactions from this central “hot spot” deposit enough energy in the rest of the fuel that fusion occurs there too–this process is referred to as ignition.  Note that during the very short period of ignition (a few nanoseconds, or a billionth of a second) the fuel is pushed inward, so its own inertia acts to impede its disassembly; hence the term inertial confinement fusion.