It was already mentioned on this forum that we have a strong connection to music, Bach, Brook and that B, A, C, H are also notes Johann Sebastian Bach himself used in his work to write his name. Some Musicians think that Bach used it as a signature and planted it deliberately in his music.
On the roof Sherlock is able to convince Moriarty that he can make him call off the killer(s).
Let's presume that there is an actual code or a number Moriarty has to transmit. Sherlock sings "I have got you".Four notes? Sherlock taps with his hands behind his back. Moriarty also tips with his left hand on his knee in 221 b.
We are supposed to think that this is the digital code. But there is no code. Moriarty invented it. And fake/doofus Sherlock did believe it? No.
What is the meaning of this tapping?

Combine music, code, tapping, emergency situation, need to communicate with someone and know that there might be no signal on Moriarty's phone. Sherlock had no signal in ASiP.
This is all nothing new. Go away if you don't want to know it. NOW. (just to be safe)


We know that there is a sniper on the stairs in the hallway somewhere. He has to have a position where he can see John (because he eventually has to kill him if Sherlock is still on the roof when John enters the hospital or gets out of sight). He also has to see Sherlock or Moriarty on the ledge because he has to see whether Sherlock jumped. You may paint two overlapping areas in your Mind palace.
John and Sherlock always use cabs. Everybody knows that. John is called away to see Mrs H. (I think Moriarty or his assistant did it). From this minute Sherlock has 20 minutes to jump. It is the metaphorical sandbag in TBB. Sherlock has to free himself in time to rescue John. This mirrors the rescue situation in TBB at the end when Sherlock comes to rescue John and Sarah is the victim.

Sherlock and the sniper know that John will rush back to the hospital to help when he realises that the phone call was a fake.
"Where did the cab drop you?" A question Sherlock asked in TBB. Look at the ground now. The cab is not allowed to stop in front of the hospital. There is a bus stop and the emergency station. The two yellow lines on the pavement begin exactly where the cab stops. John has to get out and walk. Sherlock knows the hospital quite well. And so does Moriarty. He worked there, too, in his Jim from IT disguise.

Let's see it now from the sniper's perspective:
Where is the best place to be positioned? In TBB Sherlock figured out the name of the recipient of the yellow lines on the blind banker's face by imagining the line of sight.
The best way to shoot a moving target is to be in front or behind. It is more difficult to aim when the target moves traverse the line of sight. He can't be on the same roof because there is no place to hide and Sherlock would see him. He can't be in the hospital because he has to see the roof. He can be on the left side of the hospital but then he can see John just for some seconds when he is seen coming from behind the emergency station. 
Conclusion: The sniper (let's call him Moran) is probably somewhere behind John. I imagine that John is exactly between Moran and Sherlock. Moran can see them both. Sherlock has to prevent that John gets out of Moran's sight when he is hidden behind the emergency building. So he asks him to walk back and stand where Moran can see him.
With this Sherlock turns the clock back to get some additional time to talk.

The sniper aims at John and he can see that John looks at the roof because his head is upturned. Moran doesn't need to see Sherlock, too, because he can see John's reaction.

Sherlock is sure that Moriarty has to communicate with Moran. How to call him without being able to call him directly?
If BACH is involved, music and a code, "The case that made my name" can be read litterally. "Any metal objects? Keys, mobile phones?"  It is the phone case.
Use the phone as a mirror to signal the reflection to Moran? It is perhaps the morse code used to signal BACH. It is a sunny day and we see Moriarty sitting on the ledge at the very beginning of the scene holding his phone. The picture is frozen for a very long time if you look at it.
John used the morse code in Hounds. Maybe it is now mirrored and Moriaty uses it to communicate with his assistant (Sebastian) who is also a military man (canon) with morse code knowledge.

The code being BACH would be a nice parallel to SHER in Scandal.
The whole scene looks a bit like The Empty House now because we have Moran with a gun at the other side of the road aiming through a window at fake Sherlock (who plays ordinary-Sherlock).The writers just give us the little nod to the hanging dummy in 221 b, but the real dummy is Sherlock being doofus-Sherlock.

So Sherlock wanted to show Moriarty that he can make him walk to the ledge, use his phone and signal BACH to Sebastian Moran. Would be really humiliating, wouldn't it? Similar to making a doctor fetch a cigarette pack for an addicted person.

What do you think? If I am wrong is was at least fun to invent something that fits in with some of the facts.