Let us say that I have a form to search by multiple tags in wordpress.

<input type="checkbox" name="my_tags[]" value="tag1" />
<input type="checkbox" name="my_tags[]" value="tag2" />

I want to make it so that when my plugin sees that my_tags[] is set, it rewrites the request to say

mysite.com/?tag=tag1+tag2

I know to use add_query_arg to put in the tag, and to use the query_vars filters to allow my arguments to be sent, but what do i hook into to get the arguments that were sent after the request, but before wp parses the request?