I'd never noticed it before. Jim scares me. As well he should, he's a deeply scary man. Not human; he left humanity behind long ago, wearing his body like unwanted clothes.
Allow me to backtrack. I was rewatching a favorite fan video... it involves Reichenbach, so if any feels still bother you, you might want to skip the video itself.
For your Entertainment | Jim Moriarty (2012)
Criminals are rude. At the very best, and far far worse at the other end of the spectrum. 8(
Think about Moriarty. Kidnapping people... making them into weapons, into suicide bombers. All because he was bored.
Objectifying people, making them into things. Seriously, what level of self-hate, let alone hating others, must he have possessed?
Look at his face during the tea scene in Baker Street, if you doubt me. The hatred is pretty strong, and it is the same face of hate just before Jim steps out into the pool. A face full of hatred: "I hate you, Sherlock; I will destroy you." That kind of primal that you don't see that much... most of us mask it, even to ourselves.
So very effective, so attractive to someone full of hate, finding nothing of value in life. He tested it out, and then Jim used the same method. There's wanton death, by killing a lot of people, like with the old blind woman; and then there's killing his archenemy's most beloved people. Yeah, I can see why Jim would go with that.
As a math problem, it makes sense, because Jim KNEW what level of anguish that would cause Sherlock. He could see it. The anguish of a lot of people Jim didn't even care about? Far less interesting!
Besides, to Jim, Sherlock was his toy -- his obsession and plaything. How dare Sherlock fall into the "normal people" path of caring! It's probably why Jim got lazy and let Sherlock choose the venue again, which of course turned out badly for Jim's master plans.
(Yeah, I understand the mindset. A great many intelligent people in our fandom have helped me with ttheir thoughts; thank you all.)
But the signal--
Remember how people argued over Sherlock's signal[s] on the roof? When Sherlock held out his hand to John, it signaled to the ground crew that SH was about to jump.
Dear Jim's signal must have been shaking Sherlock's hand.
No wonder Sherlock was confused. Perhaps you shook your opponent's hand before a duel, or to symbolize that a deal was on. I'd say it was both for Jim. Plus the bonus of showing his lead sniper not to shoot Sherlock, but to shoot John instead. Unless Sherlock jumped.
Dear Jim just gets creepier all the time.
Jim killing himself could've been the signal. I think it's a reinforcement. His henchman [or hireling, depending on your perspective] could've accepted that. Or he might not have known that was the start of the finale.
I can't see Jim confiding in someone, really. "This is the signal" is one thing.
He probably hammered into his man's mind: "When you see me do this, then you know to do the next step; shoot your target, unless SH does as he's been herded into doing." Doesn't matter if this was Sebastian or not. And if it was Sebastian, the odds are even lower of Jim confiding that was THE signal [death following a handshake!].
I'll catch up on blogging soon. This past month... well, it's been full of crazy, of loss, and stress.
Hope you've been having fun on the Interwebs! I hear the buzz is really strong for Doctor Strange. Bwahahah!