spark-advance control

However, of the many factors involved, probably one of the principal considerations in arriving at their final decision was the location of the spark-advance control, which was in effect the only control they had of engine output, except for complete shutoff.

Since a simple ignition switch was all that was required to stop the power output, regardless of whether a fuel-control valve or a spark-advance control was used, it must be concluded that the primary function of the compression release was to facilitate starting, and any other useful result was something obtained at no cost.