I've never seen this before, I've always known there was either GET or POST. And I can't find any good documentation.
GET send variables via the URL. POST send it via the file body?
What does HEAD do?
It doesn't get used often, am I correct?
W3schools.com doesn't even mention it.
The HEAD method is used to send the request and retrieve just the HTTP header as response. For example, a client application can issue a HEAD request to check the size of a file (from HTTP headers) without downloading it. As Arjan points out, it's not even valid in HTML forms.
HTTP method HEAD sends the response's headers but without a body; it's often useful, as the URL I've given explains, though hardly ever in a "form" HTML tag.
The only thing I can imagine is that the server may actually have been set up to validate the request method, to discover submissions by robots that for HEAD might actually use a different method than a browser does. (And thus reject those submissions.)
A response to a HEAD request does not imply nothing is shown to the user: even a response to HEAD can very well redirect to another page. However, like Gumbo noted: it's not valid for the method in a HTML form, so this would require a lot of testing in each possible browser...
For a moment I wondered if HEAD in a form is somehow used to avoid accidental multiple submissions. But I assume the only useful response would be a 301 Redirect, but that could also be used with GET or POST, so I don't see how HEAD would solve any issues.
A quick test in the current versions of both Safari and Firefox on a Mac shows that actually a GET is invoked. Of course, assuming this is undocumented behavior, one should not rely on that. Maybe for some time, spam robots were in fact fooled into using HEAD (which would then be rejected on the server), or might be fooled into skipping this form if they would only support GET and POST. But even the dumbest robot programmer (aren't they all dumb for not understanding their work is evil?) would soon have learned that a browser converts this into GET.
(Do you have an example of a website that uses this? Are you sure there's no JavaScript that changes this, or does something else? Can anyone test what Internet Explorer sends?)