1 round lag song, costs 20 mana. It is exactly what the people have said. People attacking you with passive melee and single target skills will sometimesfail to target you at all. They still get lagged for attempting the skill.
You will see "attacks some empty space near you" as the bard.
The effects are alright, I'd say around 25% fail rate, but to get the most out of the song I would strongly suggest taking the distortion opportunist edge. It is expensive, but given how often you use echoes and what it does, it is one of the best bard edges there is, and it does make people miss far more often. I'd say about 33%. The most useful part is that people who use flee /reengage tactics will often miss their openers, allowing you to sleep/fiend them more easily.

Far as I know it does nothing for area/group attacks. So using it against other bards is pretty much a waste of time.
It shines mostly against people that rely on passive melee and single target skills/spells/supps.

When I last played a bard it seemed like the ease of landing the song went down, so it was less reliable that you could distort someone within one or two rounds of combat, but that is probably anecdotal and not based on the actual code. Or people just knew to prep against me, since resist mental preps ruin a bard's day and are relatively easy to obtain.