This option determines whether holding down Ctrl will enable snapping to the guide path or prevent snapping to the guide paths. With this enabled, if you want to drag a new run and have the modules automatically snap to the guide path, you need to press Ctrl. With this disabled, Ctrl does the exact opposite, and you have to hold down Ctrl to NOT snap to the guide path.

The default is for this to be Disabled, meaning you have to hold Ctrl to move a module off the guide path.