count it or not?

[elecref]

maybe i am using too much logic. i understand that the try ends when it is successful, unsuccessful, touches the floor or becomes dead. i understand that we have a live ball in the air. so i am just trying to gain a better understanding on what makes the try end in this situation and thus the ball becoming dead? the only thing that i can use is the fact that the period end with the sounding of the horn and thus since a teammate touched the ball after the horn sounds, the ball becomes dead. any help will be appreciated.