bard charm

[kuuky]

Can someone explain to me how you charm with a bard. I understand it lasts only 18 seconds (3 ticks).
Do you need to stop the song and start it again?
Does it continually charm the mob 3 ticks at a time?
When it wears off does the previously charmed mob take hits at me and how long is it before I can recharm him?
There will probably be other questions as these get answered. Thank you