I do believe I have solved it…. so here we go!
(Sherlock series 2 ending spoilers!)
Do not make the mistake of thinking Moriarty caught Sherlock off guard. He did not. Sherlock had known for some time that this was coming. Not only that, but Sherlock picked the venue for his “death,” which gave him the advantage when planning how to fake his demise. This is the how he did it:
1.) Sherlock gets John out of the picture, by way of the fake report that Ms. Hudson has been shot. You can tell the call was Sherlock’s doing because it didn’t faze him at all (more on that later).
2.) With John out of the way, Sherlock proceeds to call in some favors from Molly and other contacts, so he can be sure everything will be ready when the moment arises.
3.) Sherlock invites Moriarty to meet him on the roof of St. Bart’s.
4.) Some time elapses at this point, allowing John to continue his false mission and Moriarty to move into position. Sherlock then goes up to the roof and meets Moriarty.
5.) Moriarty and Sherlock have their little dialogue, which is of minimal importance, except for the moment when Moriarty and Sherlock peer over the edge, when the camera had a clear view of the street below.
This is what we see:
The street below is actually quite narrow, and there is an additional one-story building between that and the main road. The main entrance to the hospital is around the corner on the other side of the building. A few people are milling about below, which is what Moriarty remarks upon.
6.) Sherlock requests a moment alone, and Moriarty leaves Sherlock alone at the edge of the roof. Although he appears to be pondering an escape, Sherlock is more likely giving some sort of signal to someone on the ground, in order for them to begin preparations.
7.) Moriarty dies, without ever looking over the buildings edge again, and Sherlock prepares to jump. Meanwhile, John arrives on the main road and starts around the corner towards to hospital entrance.
8.) Sherlock calls John, instructing him to backtrack. John winds up standing with the one-story building between him and St. Bart’s. This is a vital detail, because although the camera focuses mainly upwards on Sherlock, John can’t actually see the ground where Sherlock would conceivably land.
(Many people have noted the presence of a garbage truck below, which would have pulled into place at this point, although John does not see it.)
9.) Sherlock jumps and lands in the truck below, his fall cushioned by garbage bags. This theory is quite common on the internet, but what makes my version special is that Sherlock knew he would have a safe landing. How?
Flashback to Series 2, Episode 1: Mrs. Hudson had been threatened by the American, and Sherlock comes to her defense. Its notable, because in the final episode, Sherlock barely blinks at the news Mrs. Hudson had been shot. But what did he do before? Sherlock dropped the American out of the window. Several times. And, as you may recall, the American lands, repeatedly, on Mrs. Hudson’s garbage. The American survives the experience, to appear again later in the episode. The fact is, Sherlock was running tests, as he is so fond of doing, on how a fall from great height could be cushioned by, yes, garbage.
So Sherlock knew that he’d have a safe landing, and was able to scramble out of the truck onto the ground, where he lays down, in a puddle of blood (easily procured from Molly), and pretends to be dead.
10.) Horrified, John goes running across the road around the one story building, but before he can get a good view of what’s going on he gets his by a bicycle. This is also important, because John looses track of what’s going on, he may have even been unconscious for a moment, and when he gets up he seems to be concussed, not to mention probably in shock.
11.) John stumbles over to Sherlock and attempts to take his pulse. Because of John’s combat training, it is unlikely that he could entirely miss the fact that Sherlock is alive, but it seems probably that Sherlock is able, like a trained biathlete, to calm his heart-rate very quickly. Additionally, the Sherlock Holmes of literature used a fictional form of martial art to escape his death. It seems like a good connection to the original work would be established if Sherlock used some form of meditation to lower his pulse enough that John, who was extremely rattled and potentially concussed, would miss it, especially since John didn’t take very much time searching for a pulse.
12.) Before anyone could realize the deception, St. Bart’s medical staff whisked Sherlock away. From then on it would be a simple matter for Molly to fake his records, and Sherlock to slip off into hiding. This could be accomplished similarly to Irene Addler’s deception, or with Mycroft’s help.
Regardless, the facts stand. Sherlock jumped from the building knowing he would have a safe landing, because he’d done tests. John wasn’t in his right mind taking the pulse (I also considered that the cyclist might have slipped John some of the drug from the previous episode, but it seems more likely that John had a head injury and wasn’t thinking straight.) And Molly could easily have helped Sherlock leave the morgue.