I appear to of caused some confusion with my description of how the lane changer flipper works, so I'll try again.
The easiest way to look at it, is that if you want to change lanes you press the lane change button and you will change lanes. If you don't want to change lanes, don't press the button, and you won't change lanes.
Whilst the lane change flipper doesn't reset after a car changes lanes, it will switch back if the next car doesn't want to change lanes - or it stays where it is if the cars does want to switch lanes.
A cable connects the sensor track section to the lane change track section, which carries the signals to control the switch. Hence the sensor has to be placed directly before the lane change section.
The reason for having two seperate track sections is to cater for future enhancements, ie new lane change sections. Whilst this obviously helps to reduce the tooling costs for Scalextric, it also means that you might be able to buy a lane change section on its own without having to get a new sensor straight.
Hope this all makes sense.
Jon.