If an opponent who controls at least as many lands as you do would put a land onto the battlefield, that player instead puts that land onto the battlefield then sacrifices a land.
10/1/2009 This effect applies no matter how the land would enter the battlefield: because an opponent plays it, or because a spell or ability allows that opponent to put it onto the battlefield. Note that it doesn’t matter whose control the land enters the battlefield under. If the opponent would put the land onto the battlefield under someone else’s control (as a result of Yavimaya Dryad’s ability, for example), that opponent will still have to sacrifice a land.
10/1/2009 If an opponent puts a land onto the battlefield under their own control, they may sacrifice that same land. The player won’t be able to tap that land for mana before sacrificing it.
10/1/2009 If a land that would enter the battlefield has its own “If [this land] would enter the battlefield” replacement effect, the player who will control that land chooses which replacement effect to apply. After applying one, if the other one would still be applicable, it’s then applied.