Move your playhead to the point where your subject starts moving. In the "center" row, click on "+" sign to select the region where you want the subject to be blurred. Then click on the diamond icon. That will set a keyframe. Then move the playhead in the timeline to where your subject stops moving, hit the "+" sign. A keyframe will automatically be applied. Rinse and repeat.
Naturally, if your subject moves a lot, it is much easier to track in AE or Motion.
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