Lock out occurs when the ph of your medium has swung either too high up or low, the plant loses all ability to suck up certain nutrients. PH lock out is a bitch, I've done battle with it several times before...its my growing arch nemesis. (You are partially correct, it does tend to happen when you feed the plant too much as lots of salts build up in the soils and alot of ferts swing the ph towards being more acidic.)

And you flush by running PH'd water (PH like 3 gallons or w/e at aroun 6.8) and just continue running water through your soil until you've rinsed out all the nasty salts and such and your soil will be sitting at a nice happy ph of 6.8.That way your plant can eat and be happy again.

The generally acceptated amount for a successful flush is 3x the pot size. IE if you have a 1 gallon pot, 3 gallons of water. I've always had to use WAAAAY more then the pot size but all my ph problems have been really really strange.