By sending a tip, even if the post gets voted down to zero the author still gets something. Tips are available on the peakd interface. It is pretty much the same as sending a wallet transfer, in fact I think it is the same. https://peakd.com/hive-163399/@peakd/tips-notifications-and-other-improvements was the post that introduced the tip function.
I mean the tips are not really invisible, all anyone has to do is mouse over the little gift box Icon (tips), and see who sent one and how much.