Discussion Forum

When power sink resolves, you either pay X mana or let your lands become tapped. The lands that become tapped are not "tapped for mana". If you choose to pay, you may pay the X mana using whatever mana abilities you want to use. so if the lands are already tapped because of the web, then you can cast power sink for 1U because all lands are already tapped. but power sink only taps lands, so if they have other mana sources then the opponent can pay the X cost of power sink.

You can also tap a land for mana in responce to Mana Web's ability triggering, so if he has 6 lands out and wants to pay 3 for x, then he taps the first land, which triggers the Mana web. He then IR taps 3 more for the mana. Once the Web's ability resolves all his lands will be tapped, but it doesn't really keep them from using their lands for mana.