The horizontal is referring to the swingarm. To be able to get the swingarm in a straight line between the front sprocket center and rear sprocket center, you must be able to either compress the shock enough for this to happen. Instead of compressing the shock, you can also take the shock loose from the swingarm, or remove the shock completely.
There are two bolts that hold the shock in place.
I realize that this info was in a magazine, so it must be infallible spot-on information...but just a heads up. A full inch of play in the chain (when the swingarm is in it's tightest position) is a bit much. On most dirtbikes, 1/2" -3/4" of total up and down slack movement is more correct.
The correct setting would be anywhere between 2%-3% of the distance between shafts, in total up and down movement of the chain when in it's tightest position.
