when you scrub or play back the timeline, your video appears in the right, or timeline, side of the trim window. when you double click on a clip in a bin (in a gallery or timeline), it opens in the left, trim side of the trim window. if the trim window is in its contracted state, it expands to show you the in and out preview screens.
you can also close the trim window by clicking on the x in the top right corner of the trim window.
reopen the trim window by selecting view ] trim window from the main menu.
you can make the preview / trim window larger or smaller by placing the cursor over an edge pressing the left mouse button and dragging. if you have made the preview / trim window larger in this manner, you can make it smaller again the same way. you cannot make this window smaller than the default size.