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A cross post under the hood creates a new post on the cross posters account. There is a default beneficiary split with 90% going to the original poster, 5% going to the cross poster and 5% going to the community account.

So if you cross post a post that is past the 7 day payout it re-opens that post for rewards for another 7 days.

Yep, OK, I understand that. I guess it can help and it's a good way for some experimenting to happen to see how well it can work.

At the same time, I am thinking that it dilutes the original post - meaning that the rewards are now spread across multiple posts. If all rewards are shown in the original post, then other people can see that this post got quite a lot of votes on it so they will be more likely to open and read it.

Also, it requires that someone cross-posts so that a new 7-day payout window is started.

Some time ago someone created a website that introduced a way to vote on Steem posts indefinitely. The way they made it work is by creating a new comment from the post author, and placing the upvote on the comment. I guess this is a workaround that solves a lot of problems.