Either it's strange PTS lag, or the 20% proc works in mysterious ways. It won't trigger when you go below 20%, it triggers when it does its heal over time (every 2 seconds) assuming the heal does not bring you back up over 20%.
So you gotta survive a potential 2 seconds after going below 20% for it to work. It also does NOTHING if the heal over time brings you over 20%. I tested it with a buddy of mine. He hit me down to 14%, then we slowly waited for the heal to bring me to 21% - oh, it didn't end up triggering. It seems incredibly weak.