The analogy was taught to us all the time during my school times, was of two shelves in the library. One of them is almost empty, with the exception of a single book close to the middle. A librarian chooses to drag the book towards the left side.
Below that, is a shelf that is nearly full that has a gap one book wide, in the middle. A librarian chooses to slide the gap to the right hand side, without affecting the sequence of the preceding book… So, she moves the first book into the gap originally created, after which the second book is placed in the gap that was created then the third book into the gap, and so on to the final.
Both are a net flow of one book moving from left to right on the shelves, however the movement of one book is significantly greater than the gap between the books in the opposite direction.