JC Whitney used to sell these little caps to put on your spark plugs which claimed to solve misfiring due to oil fouling etc. Later on there was a larger device which plugged into the center post of the distributor. I remember breaking these apart and they were nothing more than a gap the spark had to jump. In a similar vein there were "Booster Gap" plugs from the major spark plug makers which had the gap built into the plug.
Spark Intensifier: "The purpose is to provide a gap in series with the high-tension circuit for the spark to jump before it reaches the plug. It is claimed that this will intensify or raise the voltage of the high-tension circuit to such an extent that it will jump across a carbonized plug in spite of its fouled conditon".
It goes on to discuss various spark plug problems (carbon leak paths) and how they could result in mis-fire. The action of the intensifier gap is explained as follows.
The Action: ...."If the intensifier gap is provided, this temprorarily acts as a dam to the flow of current, and, like flowing water, it will pile up against the dam and thus establish a greater force when released. After the breakdown of the dam, the current comes with such a rush that it connot all pass through the carbon path, and as its voltage is now sufficient, it will also jump the spark-plug gap".
Rather than the dam analogy, I have also seen it explained it in terms of ignition rise time. As the magnetic field in the coil primary collapses the high voltage induced in the secondary coil winding does not occur instantaneously but rather increases over time. Conductive deposits on the spark plug can bleed this voltage off before it can increase to a level sufficient to jump the plug gap. The spark gap reduces the rise time since the voltage (across the gap) must increase to high level to ionize the large gap. Once ionization occurs the air in the spark gap becomes conductive and current flow is immediate - thereby jumping the spark gap and the plug gap.
In conclusion, pulling the spark wire away from the spark plug is simply acting as a spark intensifier. The effect is the same regardless if the plug wire is removed from the firing cylinder or the mis-firing cylinder as long as the spark is allowed to jump to the plug.
As to the question of how one can remove the wire from the firing cylinder and not allow a spark to jump to the plug I could only speculate. Perhaps the coil secondary is center tapped and grounded or otherwise finds a ground as other posters have suggested. However, it is not stated what is done with the wire after it is removed from the firing cylinder. If it is left to lay against the engine a spark may be jumping across the rubber terminal boot to ground - acting as a spark gap.
I am not an electrical weenie however, this is how it fits into my head.
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