In the VideoTrimmer UI you have two silders – one for setting the start position and one for setting the end position:
Everytime you move one of the sliders to a new position, the content of the video player will be updated, showing you the currently selected position within the video. You will also notice that either the start or the end time (HH:MM:SS) will be updated to reflect your changes:
To fine-tune start and/or end position you can make use of the buttons PLUS and MINUS right of each slider. With those buttons you increase or decrease either start or end position through one single frame. So if you’re trimming a 60fps video, pressing the PLUS button 60 times would increase the start position about 1 second. So you see this is really for fine-tuning.
After you’ve set the perfect start and end position it’s getting time to truly trim the video by clicking on the TRIM button in the apps command bar:
After doing so, the video gets re-loaded in the editor, starting and ending at the specified positions.
Now you once again could trim it (when you see that start or end position would need some fine tuning) or you can move forward – that is either add an audio soundtrack to your file and/or save the trimmed video file to disc:
After you’ve saved the video to disc you could upload it on YouTube or share it to other UWP win10 apps installed on your system.