It creates a horizontal or vertical cut across an image, then lets the user slide the two resulting sides relative to one another.
The stated effect is to remove a section from the middle of an image while retaining its outer portions. Willison describes using it after stitching several phone screenshots into a long image, when an irrelevant section such as a prompt in a transcript needs to be removed. The source describes the interaction as a horizontal or vertical cut followed by repositioning the two sides.
The source presents Seam Cutter as a specific standalone tool rather than a general image editor. It does not establish supported file formats, image-size limits, whether processing happens locally or remotely, browser or platform support, licensing, pricing, availability terms, or privacy characteristics. It also provides no performance figures, export details, or evidence that the tool is intended for image workflows beyond the example described by Willison.