RDH- If the mower moves on level ground while the controls are self centered to neutral, you can stop the mower from moving by adjusting those knobs. If the right wheel moves, adjust the right knob in the direction that makes the wheel stop. Left knob for left wheel.
The neutral safety switches may need readjustment if the mower won't start after you make the neutral adjustments.
These switches are supposed to be activated when the controls are in neutral so that you can only start the engine while in neutral. I've seen where adjusting the neutral knobs has screwed up the function of those switches.