Once HS happens it negates all existing CC effects. You could be Stoned, Charmed, Webbed and Tripped and those ALL go away once the timer for the first effect on you has hit the magic moment. (might even apply to things like crippled, but not sure). Would be interesing if it also applied to things like Orange/Red DA effects on your movement... Then you are immune to any CC until someone casts another CC at you for a period of time. (on characters and mobs it can be upwards of 20 seconds).
It will be interesting to see if you can bork some quests by forcing CC onto you so you get the HS and then are able to ignore the next CC and make otherwise impossible attacks/events happen due to being immune to CC for a brief period?...