TCG Rulings

• You place a Spellstone Counter when the monster’s effect finishes resolving.^[1]

• You place a Counter for each monster effect that resolves, even if more than one effect is activated in the same Chain.^[1]

• If you control multiple copies of Powersink Stone, each one gets a Spell Stone ^[sic] Counter.^[1]

• Removing the Spellstone Counters at the End Phase starts a Chain.^[1]

• Powersink Stone’s effect to remove its Spellstone Counters can activate multiple times during the End Phase if monster effects continue to place Counters on it.^[1]

• If this card is negated by Royal Decree, all Spellstone Counters on it immediately vanish.^[1]

• If Light and Darkness Dragon is on the field and Powersink Stone gets its second Spellstone Counter, Light and Darkness Dragon’s effect will be negated and its ATK and DEF will return to the original values.^[1]

OCG Rulings

• You place the Spellstone Counter when the monster’s effect finishes resolving. That is, when the effect initially activates, do not yet place the Spellstone Counter.^[2]

References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5 1.6} Konami Gameplay FAQ: Starstrike Blast Sneak Peek -- Card Rulings (Version 1.0)
2. ↑ Konami OCG FAQ: At what time do you place the Spellstone Counter on "Powersink Stone"?