I think the Haitians blocking power is on all the time. He had no reason to block anything when Matt tried to use his ability on him, and ended up with a nosebleed. It's like he's a void or null spot, with the ability to create a void in others (memories, conciousness, etc.) After all, conciousness is controlled by neurons in our brains and if he slows the neurons or controls them, lights out...
wait a sec....I may be on to something here. Sylar is taking brains out of people, possibly while their neurons are still firing (while they still live). We have no reason to believe that the first guy, Brian, was instantly dead when he got bashed in the head. Also, wouldnt it be easier to take the brain of a cheerleader if they werent still kicking and screaming?
Any thoughts?
Well, most brain surgy is done when people are awake. I guess that's possible.