In an Ajax transaction, the HTTP request is sent asynchronously after the XMLHttpRequest object is created and the request parameters are set using the `open()` and `send()` methods.
When the `open()` method is called, the XMLHttpRequest object is created and its properties are set, such as the HTTP method to use (e.g. GET, POST), the URL to send the request to, and whether to send the request asynchronously or synchronously.
When the `send()` method is called, the HTTP request is sent to the server. If the request is asynchronous, the browser does not wait for the server's response before continuing to execute other code on the page. Instead, it continues to run and executes the callback function specified in the `onreadystatechange` property of the XMLHttpRequest object when the response is received from the server.
The server processes the request and sends a response back to the client, which the browser then receives and passes to the callback function. The callback function can then use the response data to update the page or perform other tasks.