How does "hash" work in the context of the Contact Form?
In the context of the Contact Form plugin, the |hash filter is only necessary if you want to dynamically add recipients to the submission.

It is there to ensure that data isn't tampered with between the time it is submitted and the time that it hits Craft.  Contact Form cares about this so malicious people/scripts don't turn your Craft install into an email SPAM relay machine.

Looks like you're using it on the hidden redirect parameter... it's completely unnecessary there, so you can remove it.