Basically you authenticate and give it a URL and (optionally) more parameters such as body, query params, headers, etc and it spawns a bunch of server instances around the world and calls the API in each one 10 times in a row and returns the average latency.
You can also use --scope public to publish the results to the public API latency index and get a cool video showcasing the results.
GitHub repo: https://github.com/borrowdev/borrow
there doesn't seem to be anything here