The post you commented on explains it:

If there is a post you would like to see curated, simply submit to @buildawhale as usual EXCEPT send 3 STEEM instead of SBD.

^^This guarantees curation but no buildawhale upvote ^^

How do you get featured in our Curation Digest?
Submit a bid to @buildawhale for 1.000 SBD or more and collect your upvote. Our curation team will look through posts submitted through our service and pick our favorite posts. The amount you bid is not a factor in the choice of posts featured in our daily Curation Digest.

^^ This guarantees an upvote but doesn't guarantee curation ^^