What is the switch on the bottom side of steering column on a 1979 Ford E250 Econline Van?

My van will not shift into reverse or park and will not start. I have replaced the neutral safety switch, checked the shift cable and shift cylinder on the steering column. While doing that is see a sliding electrical switch on the bottom of the steering column which works in conjunction with the shift lever. What does this switch do?

Read more: What is the switch on the bottom side of steering column on a 1979 Ford E250 Econline Van?