Mohtaram vagabondpiper Sahib,
Here are my points regarding ATC techniques:
1. There are two types of radars, primary and secondary.
2. Primary Radar: This is the most traditional type of radar used in the military and not used in civil aviation, its only use in civil aviation today is as a backup and a complementary technology and not as a primary option for it does not serve the purpose of civil aviation, horses for courses actually.
3. Secondary Radar: This is the system which is transponder based and used almost solely in civil aviation for monitoring air traffic. It helps identify who you are when you are flying an aircraft. ATC (Air Traffic Control) knows which aircraft is which because of the Secondary Radar System.
4. Effects of switching off your transponder: You go off the secondary radar but you can still be picked up by a primary radar. ATC (Air Traffic Control) will not be able to see you since civil aviation systems don't use primary radars. Your blip on ATC screen will go off and you will become invisible to the poor air traffic controller. Now you are virtually invisible on ATC's screen. This is probably what happened to Malaysian Airline's MH 370.
5. ADS-B: This is the new system which will be introduced in the future: ADS-B (Automatic Dependent Surveillance-Broadcast). It will make today's radar redundant. North America plans to introduce it in its airspace by 2020. It is already in place in Australia, it started working there in December 2013.
Conclusion: If you switch off your transponder, you become invisible to ATC (Air Traffic Control) since they employ Secondary Radar technology and not primary one.
Regards.