A JSONP (JSON with Padding) developer manipulates data transfer in web applications, overcoming the same-origin policy in web browsers. They use JSONP to request data from a server in a different domain. The developer writes scripts that request JSON data, wraps it with a function, and then sends it as a callback function. This technique allows the exchange of data among different domain servers. The JSONP developer must ensure the data is secure and correctly integrated into the web application. They troubleshoot issues, debug code, and work with APIs to enhance the application's functionality.