Some browsers, clients, networks, firefwalls, etc. don’t allow HTTP methods beyond GET and POST.  With Grails you can get around this by POSTing an additional parameter ‘_method‘ which will be used to override the request’s HTTP Method/Verb.

If you post with an additional Parameter of ‘_method‘ (or the X-HTTP-Method-Override header if the ‘_method‘ parameter isn’t present) Grails will internally route it correctly using the value of that parameter (or header).  This is helpful when doing RESTful services or clean URLs that map to HTTP methods.

This is helpful if you are using a client that restricts HTTP verbs like Flex used to (still does?) or other browsers.  In fact the <a title="g:form Tag" href="http://www.grails.org/doc/latest/ref/Tags/form.html">&lt;g:form&gt; tag</a> will add the extra _method parameter for you if you use a method other than GET or POST.  This is something to keep in mind when doing Ajax or using alternative clients.

Hidden in chapter 13 of the documentation is one paragraph about this:

<g:form controller="book" method="DELETE">
  ...
</g:form>

Grails will send a hidden parameter called _method, which will be used as the request’s HTTP method. Another alternative for changing the method for non-browser clients is to use the X-HTTP-Method-Override to specify the alternative method name.