He claims that the plastic on the original H-bomb test was placed around the
inside of the bomb casing, not on the fusion core itself as I've read
elsewhere. The X-rays from the initiator turn the plastic to plasma, which
re-radiates X-rays onto the U-238 tamper, creating a rocket effect which
implodes the fusion core. This also compresses the fission trigger in the
middle of the fusion core, which explodes, compressing the fusion material
from the inside as well, and igniting it.
Interestingly, he also says that most of the yield in that test came from
the U-238, as the fusion byproducts converted it into other, unstable,
elements which then fissioned.
While I can't vouch for technical accuracy, I'd recommend both of Rhodes'
books to those interested in nuclear weapons technology.
Mark