Video Master FX, is a new feature for Serato Video users. This lets you apply video FX to the master video output as well as per channel, giving you another powerful and simple way to add interest to your sets.
Make sure you update to Serato DJ 1.7.4 to take advantage of this feature in Serato Video.
This feature is visible in the bottom right of the Serato Video panel. You have two FX slots available which will apply your selected Video FX to the master output.