MTG Salvation

If one player who has exactly 50 life uses Aetherflux Reservoir on another player who has 40 life, is the game a draw?

No. The player activating Reservoir would die. They put the ability on the stack and pay 50 life as a cost (so they lose 50 life right away). Then, the ability would be on the stack waiting to resolve to deal 50 damage to the opponent, but since they are at 0 life and their opponent is at 40 life, they would lose before the ability resolves.

Quote from The Codfather »

If one player who has exactly 50 life uses Aetherflux Reservoir on another player who has 40 life, is the game a draw?

No the player at 40 life lives because the other player will die which removes the trigger from the stack.

Paying life is an activation cost. The activation goes on the stack and you lose before it resolves. I am pretty sure that in multiplayer, once a playher loses the game, all of their cards and spells are exiled from the game, so even if you were to activate it as a some sort of concession, the ability still would not resolve and kill your target either. You need to be at 51+ life.

Sydri, Galvanic Genius can make it into a creature and give it lifelink. If you can get to 51+ life and use Sydri, if can activate, deal the 50 damage, then gain that life back to use it again and again.

I guy actually tried to do this to me online and he died from the activation cost acting immediately (he was on 50 life), so he was dead before the 50 damage could be resolved. Once a player is dead, everything about them is removed from everywhere in game, including triggers and stack.

Quote from Drain Life »

Sydri, Galvanic Genius can make it into a creature and give it lifelink. If you can get to 51+ life and use Sydri, if can activate, deal the 50 damage, then gain that life back to use it again and again.

Tamanoa accomplishes about the same thing.