Simple, clear, useful. If you left a comment before you casted a down-vote, you can cancel it without editing the question.
Inspired by litb's proposal on that matter.
Great idea. I unconsciously stopped downvoting after the downvote-lock window was introduced. It's just too easy to miss some detail that totally changes the question.
The downvote dialogue was one of the great learning experiences of Stack Overflow. You'd think-- this is WRONG and downvote it. Then someone else would explain that in fact, you were wrong. This kind of useful dialogue has all but disappeared.
Your suggestion would require a special kind of comment that can only be added with a downvote. Otherwise you could comment "+1 excellent idea" to explain a downvote :)