In January 2014, Steven Moffat provided some insight in an interview on how Sherlock faked his death:
As we keep saying to people, I do
think there was -- I can now say this -- a measure of nonsense going
on. He's falling off a building. He has one objective -- that is, not
to hit the sidewalk. That's it. That's the only thing he can do.
Didn't anybody notice that there's a whole building between John and
the point of impact? So obviously something got in the way. That's it.
It wasn't going to be anything else. The TARDIS wasn't going to fly
in. That wasn't going to happen."
Since Moffat's explanation best corresponds with the final explanation provided in the episode itself, I think the official answer is--disappointingly--that Sherlock jumped on the airbag shown in @howlers answer.
His lackluster answer fails, of course, to explain how the professional sniper was fooled, who apparently completely misses a staged jump with an enormous, brightly-colored airbag and half a dozen men.
ANDERSON: And what about the sniper aiming at John?
SHERLOCK: Mycroft’s men intervened before he could take the shot. He was invited
But according to The Reichenbach Fall, on the roof:
JIM: Okay, let me give you a little extra incentive.
JIM: Your friends will die if you don’t.
JIM: Not just John. (In a whisper) Everyone.
SHERLOCK: Mrs Hudson.
JIM (in a whisper, with a delighted smile): Everyone.
JIM: Three bullets; three gunmen; three victims. There’s no
stopping them now.
JIM: Unless my people see you jump.
So, I suppose it's just lucky that while Sherlock was hearing this revelation on the roof for the first time, Mycroft was able to find and neutralize the sniper. Thus, he was able to anticipate the sniper being there and successfully coerce him to call off the other snipers. It's a shame the only eyes he had on Sherlock was a single hired gun rather than the multiple people ("my people") he claimed needed to see Sherlock jump.
Further, if we assume Sherlock was surprised by Moriarty's suicide, one must wonder why any of the corroborators--including Sherlock--could think this would escape the notice of Moriarty himself, who expected to watch Sherlock jump to his death. He spoke the following words to John, which seems a silly line to lie about:
SHERLOCK (voiceover): But the one thing I didn’t anticipate was just
how far Moriarty was prepared to go. I suppose that was obvious, given
our first meeting at the swimming pool – his death wish.
Put another way, if Moriarty didn't kill himself, while John's sniper still would've been neutralized, Mrs. Hudson and Lestrade's wouldn't have been.
I think Moffat got outmaneuvered by the most fanatic viewers, and conceded early on before filming there were more fitting theories out there, but they were unable/unwilling to pull it off in the timeslot and writing:
As to the theories that were presented online, Moffat says he is
aware of them, but that for the most part, the audiences explanations
are far more complex than what the show ultimately needed. (emphasis mine)
"Needed" being the operative word, as there are many people perfectly satisfied with this explanation and a much smaller minority who are severely outraged at its incompleteness or implausibility.
Going deeper into the meta, I think Moffat knew he was denying us sufficient closure. As we as viewers find holes in the story and receive no answers, Andersen finds holes in the story and Sherlock abruptly disappears, leaving Andersen to completely break down. Of course, Sherlock fanatics are Andersen in this metaphor. /fanaticism