The most common form consists of a handle, shaft and head. The head is usually forked with one side of the fork flattening out and becoming a blade and the other side forming a small point. In some designs the blade side then tapers back to a point to allow easier insertion in tight stitching.
In use the blade is inserted into the seam underneath the thread to be cut. The thread is allowed to slip down into the fork and the tool is then lifted upwards allowing the blade to cut through the thread. Once the seam has been undone in this way the loose ends can be removed and the seam resewn.