To animate a blur mask for content with uneven camera or object motion, you can add additional keyframes to ensure smooth and accurate animation. Follow these steps to achieve the desired effect:

Unlock Keyframes: 

As in the previous tutorial, make sure you unlock the video effect tools' keyframes by pressing the padlock button so that the shape can animate.

Adjust Initial Shape: 

Begin by adjusting the shape, scale, and position of the mask to cover the desired area in the first frame of the shot.

Animate from Start to End: 

Animate the mask from the first frame of the shot to the last frame. The mask should follow the subject as closely as possible.

Identify Uneven Motion:

'While playing through the sequence, observe if the mask lines up correctly in the middle of the shot. If the mask does not accurately track the subject's movement, additional keyframes are needed.

Add Additional Keyframe: 

Move the current position bar to a position somewhere in the middle of the shot where the mask's animation is not accurate. Click on the "Insert Control Frame" button to add an additional key control frame. The left-hand window will become the current keyframe position, and the timeline subtrack will get a keyframe tick that can be adjusted over time.

Adjust the Mask Position: 

Move the mask into the desired position based on the newly added control keyframe. Play through the video to ensure that the mask animation is accurate throughout the shot.

Add Fade Effect:

To avoid the blur effect popping on at the start and end, add a fade effect by adjusting the upper corners of the mask. This will create smooth transitions at the beginning and end of the blur effect.

By using additional keyframes and adjusting the mask position and scale, you can achieve a more accurate and smooth animation for blurring content with uneven motion.