Downvotes on posts/comments older than X time affect the downvoters karma the same way as they do the downvoted.

Downvotes made after X time from the original posting affect karma at a rate of y%

Downvotes from users with karma below X don't affect the downvoted's karma score

All of these are made on the assumption that malicious downvoters are engaged in a E-Peen measuring contest using Karma as the measuring tool.