Dance of Derrac the Dauntless
Dance of Derrac the Dauntless is a chant that ticks 6 times over the course of 18 seconds. One tick happens every 3 seconds. Each time it ticks it applys 10 seconds of invulnerablity to the bard and the members of their group. There is also an initial tick when the bard starts the chant. The reason the chant does not refresh itself on the bard is because they are flagged with effect 'invulnerable' and thus immune to thier own power. This is also why there is a 2 second down time. The chant ticks at 9 seconds, then falls at 10. It is not picked up again until the 4th tick which occurs at the 12th second. Also, it means that, assuming you time everything right, every character except the bard could have a max of 28 seconds of invuln. In practice however it usually works out to about the standard 20.
Also, when a character other than the bard casts a power or attacks, invuln drops for a brief period of time. It will then be picked up again once the next tick hits somewhere between 0 and 3 seconds later. This new tick will grant the full 10 seconds of invulnerability, assuming the character does not cast or attack again.