You put the IOU in circulation by selling it or by giving it away. (I don't know what's so hard about that.) The point is that the act of writing the IOU does

You put the IOU in circulation by selling it or by giving it away. (I don't know what's so hard about that.) The point is that the act of writing the IOU doesn't create any new cars. And likewise the act of destroying the IOU, at any point in time, and regardless who's holding it, doesn't destroy any cars either. This simple example shows that no actual resources (i.e. capital) are destroyed as a consequence of IOUs being destroyed. If you still don't understand why this is the case, I don't know what else to say to you.