The shockwave increases the fuel density to the point neutron created in spontaneous fission are held within the material for multiple mean-free paths. When these neutrons are captured, they create more fissions. As this occurs, the temperature and pressure at the center of the pit also rises. If you inject the core with DT fuel, these are sufficient to cause fusion which liberates more neutrons which can further contribute to the fissioning process. Hence the term Thermonuclear.

Timing of this process is measured in shakes, which, if memory serves me correctly, is on the order of 10s of nanosecond. I believe breakaway occurs on the order of milliseconds. This is from faulty memory. If you want, I can verify the numbers.

Dave M