Tested yesterday and it seems it's first damage done to lana'thel after the tanks at the pull. But it seems there is a minimum requirement. A hot ticking for 500 hp won't pull the bite for exemple, but successive ones without any damage done will.
We had our hunter shoot the first behind the tanks, he fd'd and had 0 tricks on him and still got the bite. Tested multiple times with different hunters. Worked 100%