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When a potion is inside at least one slot of a potion belt and is consumed, it has a 30% of applying another potion's effect within the same potion belt without consuming it. Since the Potion Belt only has a chance of activating from a potion being consumed, the Potion Belt effect can't trigger again from it's own effect, neither from a potion's effect being applied by the Potion ⭐ mechanic.
If a Potion is in two Potion Belts, their effects apply separately, for a 9% chance of both belts triggering, two 21% chances of one triggering but not the other, and 49% chance of neither triggering. The average becomes 60% extra potion triggers.