Two problems with what they say in that video.
Marking the insulator like that will encourage tracking/flashover of the H.T. to earth. You may have done that with a soft black pencil to sabotage someone in the past...........
Spacing the plug back out of the head like that will increase the combustion chamber volume (very slightly) thereby decreasing the compression by a tiny amount. Obviously the converse is also true, any old Formula Ford/Vee racers here?
There's probably more to be gained by cleaning and setting the gaps properly.