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When you have three clips side by side in the timeline, you can perform specialized editing techniques called slip edits and slide edits. Slip and slide edits are unique to timeline editing.
In a slip edit, you change both the in point and the out point of a clip at the same time without altering the adjacent clips. It's as if you're viewing part of the clip through a space between the two other clips; when you slip the center clip back and forth, you get to see a different part (Figures 7.22 and 7.23).