In this picture, the slip ring on the left shows the problem caused by using brushes which are too hard and/or springs which are too strong. The insulator wears considerably quicker than the brass and after a relatively short time, the brush wears a channel in the insulator. As a result, the brush has to bump up and down the step created at the start and finish of the brass segment.
The slip ring on the right shows the problem caused by using brushes which are too soft. A layer of carbon has been left all round the slip ring. With a twin, when one cylinder is under pressure on the compression stroke, the brass segment will be under the brush and pickup for that cylinder but, being an electrical conductor, the carbon deposits provide a track around the slip ring to the other brush. The high voltage will always follow the easiest path and so the spark is most likely to appear at the spark plug on the second cylinder which is on the exhaust stroke and therefore at a much lower pressure.