Al-Asiri's first step was to pretend he was leaving his terrorist ways behind him and ask for a face-to-face meeting with Bin Nayef. Then he would walk into Bin Nayef's office, shake his hand, press a radio detonator, and unleash the mother of all farts. And to be fair, everything went perfectly -- al-Asiri showed up and went through all the security checkpoints without any sort of problem (yes, there are some places no guard wants to go). He met Bin Nayef, shook his hand, and then, in the words of Bin Nayef himself, "he surprised me by blowing himself up."
The problem was that it really is hard to know exactly how much anus bomb you need in order to detonate a room (there isn't a Mythbusters episode about it ... yet). At the end of the day, they probably were constrained more by the physical limits of the average human butthole than anything else -- it turned out that an asshole can't hold enough explosives to do the job. Al-Asiri's abdominal cavity absorbed all the force of the blast, so Bin Nayef merely got splattered with little bits of terrorist as the man he'd just shook hands with popped like a balloon right in front of him.
AFP via BBC
"He who exploded it ... wait, I had something for this."
At that point, Bin Nayef presumably stood there, hand extended, thinking he'd suddenly mastered the art of exploding people with his mind, like in Scanners.