Minecraft wiki entry for Player_versus_Player, associated text: For the buffs, Strength and Speed II are self-explanatory, while Turtle Master is a situational potion. In crystal combat, there will be situations where you are boxed in with the enemy in either obsidian or bedrock. In other words, there is no possible way for an end crystal to be detonated to either player's advantage and they would be stuck in a 1x1 hole made of obsidian (or part obsidian, part bedrock) In such a situation, both players would try to crit each other out, but drinking a turtle master potion grants you the powerful effect, that being Resistance IV, making you almost invulnerable to your enemy's attacks. Slowness really doesn't matter here, as you're stuck with your enemy. These 20 seconds of greatly reduced damage allows you to win the crit-trade in the hole, and with it, the fight.