I know him from T6 legit community, and trust me, that guy plays alot lol..
like, im leaving to univ, let my bot run, hes there, im back, hes there.
Im going to sleep, come back, and he's still there, asking for groups and messing around on the chat, lol...
There is a checkpoint right before the Malthael boss fight. It's where you would respawn if you fought Malthael and died.
Also, if you were to leave the game after getting the checkpoint, you can create a new game at that point, as long as you haven't killed Malthael.
Many super high paragon players are exploiting this checkpoint by using a second player who never actually kills Malthael in order to maintain the checkpoint.
I don't do these so I am sketchy on the details, but...
In short, PLAYER ONE will run through act 5 "Kill Malthael" quest until they gain this checkpoint. PLAYER ONE then invites PLAYER TWO into the game.
Once PLAYER TWO is in game, then PLAYER ONE leaves before the boss fight. PLAYER TWO kills Malthael.
PLAYER ONE creates a new game and still has the checkpoint before Malthael fight.
PLAYER ONE invites PLAYER TWO again. PLAYER ONE leaves game before the boss fight. PLAYER TWO kill Malthael.
Why do this, you might ask? There are posts everywhere from 4-6 billion exp per hour doing this.