By Law you must have a tranfer switch. If you back power , the meter runs backwards. Thats ok till they switch off the line. Then you heat up the line . so your 120 goes to the first transformer to 1100 volts , then to the sub at 66kv and so on. They do what is called chaining , or shorting you system to ground. They will also run a line impedance line thing and track you down. Bingo bubba you have a cop at your door in no time.
The reason is that you can kill some one working on the up leg of the system. Note in U.S.A and most countrys its a federal offence.
Please think safty first, the person you fry may be you.
Also note if you hook it up wrong the frequency is out of phase , it can bend you gen set like a piece of paper. Think of it as a 100 hp motor truning you gen set, the wrong direction.