What Silverwizard said. To give an example, let’s say you’re fighting a man at arms with 6D of armor on his chest but no armor elsewhere. You make a Sword test to strike him. It’s Ob 1, and you score 4 successes. It takes 1 to hit, so you’ve got 3 left over. He says he’ll take it on the chest. You spend one to move it away from his heavily armored torso and on to his unarmored head. That costs 1, so you’ve got 2 left over. You spend those 2 going from Incidental to Mark.
If you had only scored 3 successes, you would have had to spend one to hit and then choose: An Incidental hit to the unarmored head (1 to hit, 1 to move, and 1 that you couldn’t do anything with which is discarded), or a Mark hit to his armored torso (1 to hit and 2 to go up to Mark with nothing left over to move).
You can think of it as: 1 extra success lets you move the hit, 2 lets you choose to move the hit or go up to Mark (or move it a lot, like from legs to head), 3 lets you move the hit and go up to Mark, 4 let’s you go up to Superb or move the hit (a lot) and go up to Mark, 5 lets go to Superb and move the hit, 6 lets you go up to Superb and move the hit (a lot if needed).