It sounds like they figured out how to build a fusion bomb without the need for an atomic bomb primary. This would be a weapon with no radioactive signature, capable of 15 megatons of yield, a proliferation nightmare, at best.
It seems that the conjectured Ripple design uses some sort of wave-shaping barrier between the primary and secondary to convert the single massive spike of x-rays into a series of pressure waves that, via constructive interference, are able to compress the secondary directly without the need for the intervening tamper.
Being able to do direct compression this way means you can do away with the weight of the tamper, and since you're applying the force directly you can get away with a much smaller primary (e.g. a 15MT device with a 15kT primary gives you a 99.99% "clean" design).
Still requires the primary though.
Freeman Dyson in "Disturbing the Universe" says they worked on this concept but didn't achieve it, and it didn't look feasible, at least when the book was written. The device in the article had an atomic primary.