Perhaps the mechanism is to explode the bomb close enough to the surface of the asteroid to heat it with the x-ray pulse and cause ejection of debris from the s

Perhaps the mechanism is to explode the bomb close enough to the surface of the asteroid to heat it with the x-ray pulse and cause ejection of debris from the surface--that would impart a reaction force on the asteroid. That's suggested in this article: https://www.llnl.gov/news/nuclear-impulse-could-deflect-massive-asteroid https://www.llnl.gov/news/nuclear-impulse-could-deflect-mass...