Like the title says, if you want to upvote something on KBin, you should use the Boost link, not the upvote button (Why? Don’t know…)
The upvote button doesn’t seem to do much, but Boost accomplishes what Reddit’s upvote did. So if you’re looking to encourage a post, use the Boost link.
If I understand correctly, when a Kbin user boosts a Lemmy post, it will work like an upvote, bringing it up. But it wouldn’t affect anything for Lemmy users. But when a Lemmy user upvotes a post from a Kbin user, it will just be shown as an upvote but not effect reputation or anything, but will upvote it for lemmy users.