Unwind has a twofold meaning. This book is set after something called the Heartland War, which, according to "The Bill of Life" means that "Between the ages of thirteen and eighteen, a parent may choose to retroactively 'abort' a child on the condition that the child's life doesn't 'technically' end. The process by which a child is both terminated and yet kept alive is called 'unwinding.'" (Introduction). Well that's an unsavory thing for our title to reference, but so it goes.
It seems that the government never learned from the comma incident in the Second Amendment and uses unnecessary quotation marks and a misplaced modifier. Hey kids, here's your loophole: Your parents have to be between the ages of thirteen and eighteen in order to unwind you. Boom.
Grammar aside, if a child is to be unwound, they are colloquially called an Unwind. So there you go, the title of the book. It is both the identity of our main characters and what they're running away from, making it a complicated concept.