Zero keys let you define a point in time where you want the layer’s animation to have no offset from the original animation. Using Zero keys, you can demarcate the time range where you want a layer’s animation to start and end.
For example, if you have an animation sequence of 50 frames and you only want to modify the animation between frames 20 and 30, set Zero keys at frames 20 and 30 to bracket that part of the timeline. Create your modified animation between frames 20 and 30, then play your result animation. The original animation plays fully before frame 20 and after frame 30, with your modified animation playing fully in between.
Set a Zero key by clicking the Zero button in the Key Controls window. See Zero key.
When you set a Zero key, the object(s) you are keyframing may move if it is not at its “zero” position from the original animation.
