To apply a video filter only to a part of a video click Video Filters > Apply between. A dialog will open with the following fields:

Enable In Point - If this option is checked, the filter is applied starting from the frame (or time) position you specify. Click the Current Frame button to apply filter from the currently previewed frame.

Enable Out Point - If this option is checked the effect is applied up to the specified frame or time position (inclusive). Click the Current Frame button to set the out point to currently displayed frame.

Apply to Highlighted Files - Video filter is applied to all selected files in the video list.