Look people it's really not that hard, is it?
Holmes, after he jad been summoned to the roof the first time, while watson sleeps, arranges someone to call to distract Watson by sending him off on a fake errand. After all, he knows he is safer on his own and he is going to have to die. He'll need him back later as a fake witness.
Holmes verbally battles mortiarty on the roof till he realises he has no choice but to fake his death. He positions watson very carefully (note he tells him exactly where to walk and tells him where to look) so that when he jumps his eyeline will be obscured by the truck at the point of 'impact' - when he lands safely on some sort of cushion.
The people at ground level are in on the trick (Mortiaty even notes that Holmes now has an audience.) They're the ones there to set up and clear away the cushion and get Holmes prepped for Watson. They're probably friends of Molly, doctors or medical staff who getting blood for his hair and drugs to surpress a heartbeat incase Watson took Holmes' pulse). They've got the cyclist there to slow Watson down by running into him.
I wish I was clever enough to work all this out, but I'm not. It's the same trick (but from ground level) as used in Johnathsn Creek's "The Problems At Gallows Hall". All apart from the heart stopping bit that is, but that's a common trope (e.g. Spooks series 7)
Mark Fowler