When the post is approximately three days old, CR takes inventory of the post's discussion thread. Replies upvoted by the OP are eligible for extra rewards. This way, you, the OP, choose which comments get rewarded, and you can skip spammy comments if needed.
Are all comments rewarded, but upvoted comments rewarded more? Or are only upvoted comments rewarded?
Only the comments chosen by the blog author are rewarded by CR.