Gray Fox has it right with the tool-tip.
Corps and/or Army Stacks in the same Chain of Command, starting in the same region may elect to use the Synchronized Movement SO.
Each Corps and/or Army Stack must have the SM SO selected.
Then you only have to plot the move of one of the stacks using SM, and all other stacks in the region will automatically have their moves plotted exactly the same, AND their movement will be at the rate of the slowest stack using SM, so that all stacks arrive in the target region at the same time.
The question about the Traffic Rule is a valid one. I honestly have no idea, whether it would affect SM, but I'm guessing, from ACG's report, the TR probably has precedence over SM. First the moves are plotted, and then during movement, one of the Corps or Army Stacks is slowed down. So it would kind of negate SM somewhat, depending on the circumstances.
Honestly, I hardly ever use SM. I'm generally relying on MTSG to bring Corps and Army Stacks together in a battle. This way I can cover a wider set of regions, to prevent enemy stacks from filtering around my flanks and on to my lines of communication
I hate it when Athena manages to get some pesky division around my flank, and I have to put together a force of demi-corps to try to run it down and prevent it from doing any damage