Outline
This form will allow you to construct URL's which can be embedded within a web page which allow direct link through to specific resources, specifying which IdP should provide the authentication.
If the user has already logged in via Shibboleth, then they will pass through. If the user has not logged in, they will go to the specific IdP in the form bypassing any WAYF.
Why?
One question you need to ask is why you want to do this, a common reason is to guide your users straight to your IDP without them having to touch the outside world which is most useful for url you may embed with your library or internal portal systems where the IDP destination is obvious.
Other pages on the topic
https://gabriel.lse.ac.uk/twiki/bin/view/Projects/WayfLess