The technique of using only one slice of bread for a sandwhich. User would place sandwich filling (ex: ham or peanut putter) on single slice of bread and the proceed to fold slice of bread in half vertically. Or bend the bread back, hence bend back.
"Want a turkey sandwich?"
"I'm not really hungry enough to eat a whole sandwich. How about just a bendback?"
"Oh look, your Daddy made you guys jelly bendbacks for lunch."