[In case her explanation on the threads wasn't good enough, her smile grows as she reaches towards him. Might as well show her god ability first hand.
Picking something out of the space between them, she lifts it up slowly. With the contact of her finger tips, the glowing thread appears between them as she twists it gently around her index finger. It may be an odd sensation for Lavi. Like someone is walking over his grave or touching something deep inside that they shouldn't be.

The thread is long, whirling around them both but if he looks down, it seems to be coming straight from his chest]

It's this. I can see anyone's though and follow it to them. People that disappear from here....their threads do to. Or if someone is super hurt, the thread shows it too.