Such an effect has been requested in the past in the COW's AE Expressions Forum. The trick is to precomp the video and then use Time Remapping in the 'final' comp to control the playback of the video. Then it's a matter of linking the results for Motion Sketch to Time Remapping to get what you want. Again, head on to the
COW's AE Expressions Forum and conduct a search there.
HTH
RoRK
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