What does calling busboy.end(req.rawBody) do?

Question

I'm working on an image upload using express and busboy. It currently works, but if I remove the last line busboy.end(req.rawBody), the code will eventually timeout. Is this the same as the Node .end() method, and if so how is this call passing the data to / actually initiating the busboy work?

const busboy = new Busboy({ headers: req.headers });

                // Define busboy event listeners
                busboy.on("field", (fieldname, value) => {
                    // Process fields
                });

                busboy.on("file", (fieldname, file, filename, encoding, mimetype) => {
                    // Process file
                });

                // Call busboy with rawBody of request
                busboy.end(req.rawBody);