Since Cata we have had bosses that engage an extra mechanic to fights. Whether it be a bar we don't want to fill, or some button that we need to press to save ourselves. If you haven't Encountered this button yet there are 2 scenarios. You're doing the mechanic wrong, Or you never set up the button with Bartender4.

This tutorial will show you how to position your Extra Action Bar so that you can see the button you NEED to press

1) Opening Bartender4 -- Type /bt and hit enter

2) Click "Extra Action Bar" in the left menu

3) Either Check, or Confirm that Enabled is Checked

4) Click the tab titled "Positioning"

5) Click the button "Reset Position"

6) Drag Bartender to the side by click and holing where it reads Bartender4 at the top of the addon and uncheck the Lock in the top left corner of the addon

7) See that green box in the center of the screen that reads "Extra Action Bar"? Click and drag that to a place that you will see it in combat and be able to click it quickly

8) Click the Lock Button once more and close the addon. Congratulations! You've setup your Extra Action Bar and now you will be ready to click it when you need to.