I gotta tell ya Hazy it's hard for me to believe that you would actually tell someone, let alone on a public forum, how to do something that's not only totally against NEC rules but is also dangerous and life threating.
Opening the main breaker and backfeeding a circuit may work for you but it's also dangerous and illegal to do. That's the reason for the installation of a manual/transfer switch per code.
"If you forget to shutoff the main you can KILL somebody. Not just hypothetical, but actual. A search of the internet will reveal stories of linemen getting killed.
Some who do backfeed try to justify it by saying they won't forget to shutoff the main, and linemen should be properly grounding and testing before they handle the wires. Well fine, but that just leaves you 2 mistakes away from killing someone. Both of those mistakes can and do happen.
A transfer switch, by design, does not allow the generator to backfeed through the main. It doesn't rely on you to remember to switch anything. The transfer switch mechanically either allows your selected house circuits to get power from the main or to get power from the generator, but not both at the same time. It is dummy proof.
Another reason to have a transfer switch is so you can manage your loads. For example, if you have a 6 circuit transfer switch. One leg of the 240 feeds circuits 1-3, and the other leg feeds circuits 4-6. You balance the draw when you assign those circuits, and you have 2 meters on the transfer switch to view the draw off of each leg. In order to run a 240, you tie circuits #3 and #4 together so you are drawing from both legs.
Bear in mind that utilities take back feeding very seriously. If they find voltage on a line which is supposed to be dead, (1) they will search the neighborhood for houses which are lit up, ask to see your transfer switch, and if you don't produce, (2) they will report you to code enforcement.
If one of their men is injured, skip step two, and go directly to lawyers. The burden of proof is on you, and your liability is nearly unlimited, to the extent of your net worth"
Bob, yes the transfer switches are expensive but what price do you put on a life, sure hope you think it's worth more than the price of a transfer switch. Of course you're still free to do whatever it is you want to do, just didn't want you or anyone else to not know the ramifications of your decision. Here's a link that'll tell you about them and why you should hire a licensed electrician to install it for you. Hope this helps
http://www.handymanclub.com/document.asp?cID=55&dID=795