Post promotion question for Steemians who may know the answer!

I've been on Steemit for 5 days now, and I've basically just been finding my way around, reading and watching tutorials and experimenting with various features.

Something I seem to have missed, is how promoting ones posts works. I bought $20 worth of SBD for the purpose of testing out how post promotion works. What I've noticed is that, when I promote a post for 1 SBD, the post immediately receives a 0.54 upvote, and I get a message that I've sent 1 SBD to a profile called "@null".

Who can explain how post promotion works, or point me to a resource that explains it in simple terms?

Thanks in advance.