Just to clear this up for you, Boonman is stating the same thing I did. Each of the cylinders on your shee fires every revolution and 180 degrees from each other. The coil fires both spark plugs at the same time and every 180 degrees also, thus making no difference which plug wire goes to which plug because the wires are tied together inside the coil. Let me try to put it another way and keeping in mind that both plugs fire at the same time and every 180 degrees. Lets also state for argument's sake that the timing is fixed at 10 degrees before top dead center (TOC) meaning that the plug will fire 10 degrees before the piston reaches the top of the stroke. Keep in mind again that your pistons are 180 degrees off from each other, meaning that when the left piston is at TOC, the right piston is at bottom dead center (BOC) or at the bottom of the stroke. OK, having all that in mind, when the engine is running and one of the pistons is on the way up, the plug for that cylinder will fire 10 degrees before TOC. At the same time, the piston in the other cylinder is on the way down and reaches 10 degrees BOC and that corresponding plug just fired too. Fireing a the plug on the downstroke is completely useless but it was done this way, like Boonman was stating, simply because it was the easy way to get the job done. I hope this cleared it up for you. There is no theory to it, it is the way it is.