反向修剪Backtiming

反向修剪（Backtiming）是先把素材插入时间线、再指定它的结束位置的编辑方式。

• 通常的做法：先给源素材打一个 Out 点（指定结尾帧），不放 In 点。
• 把素材拖进时间线后，两个 Out 点对齐，素材会反向修剪到时间线 In 点所标的位置。
• 适合"先定结尾、再往前推"的场合，比如让镜头恰好结束在某个点。

Backtiming is an edit where you place a clip in the timeline first, then specify its end position.

• The usual approach: mark an Out point on the source clip (the ending frame) and leave the In point unset.
• When the clip is edited in, the Out points line up and the clip is backtimed to the In point marked in the timeline.
• Useful when you want the shot to end exactly at a certain frame.