It's a crowdfunded service where a sponsor account sends a memo with 1 HIVE (or more) to @steembasicincome plus a message containing the name of the sponsoree.
By rule, the sponsor and sponsoree should be a different account. Once processed, both the sponsor and the sponsoree accounts gains Hive SBI units equivalent to the HIVE sent.
If you already have Hive SBI units, then you will have free upvotes forever. The actual upvote is based on your HSBI units, it resets after upvoting your most recent post. If unused it will accumulate until a new post is published by you.
It is a trusted service here on Hive, my personal account has 161 units already sponsored by other people over the years with guarranteed upvotes from their team.
I see because I receive some SBI unit from poems I wrote in blockhainpoets (hope I spell the group name correctly) maybe the unit is still small that's why I haven't seen the upvote for quite awhile.
Thank you for simplify the explanation I think I got it now
I checked your info, looks like you've been sponsored already with 7 HSBI.
https://www.hivesbi.com/userinfo/?user=caelum1infernum
Check the upvotes on your posts if you see: steembasicincome, sbi1, sbi2, ..., and so on. Depending on their VP and RC their curating accounts will do shifting.
I see thank you for checking for me appreciate it. You're correct I did have an upvote from them in an old post