I just encountered one in my Ranked team.
They basically don't do much, beside dying and walking. They stand on base for a minut or so, then walk to the lane. Die. Then walk randomly in the map, do nothing. Stand still for a minute. Walk again for a bit. This continues for the rest of the game. They don't farm, help others, chase, kill, ward or do anything that contributes to the team.
They won't get punished for being afk, because they still walk sometimes.