Overflow and Spin
Anytime the GM determines that you have more Shifts than you need (i.e. you’ve got Shifts that are about to be “wasted”) you can turn them into Overflow.
Overflow basically means you take all those shifts, and describe a similar/follow-up action related to the original action. Depending on your description, you can just take those shifts and directly apply them.
This is called a “Supplemental Action” and the most common usage is to tear through swarms of Minions!
Any time you have 3 (or more) leftover Shifts after performing a Defensive action of some kind, you gain Spin. Succeeding in a Defensive action by a margin of 0-2 has no further effect other than usual (0 was barely a success, 1 looked workmanlike, 2 looked effortless).
You may choose to apply those Shifts of Spin as a bonus to your own next action or as a penalty to your opponent’s next action (adding or subtracting Shifts). Basically you defended so well, you’re now dictating the terms/pace of the conflict.