Winter Orb

Oracle Text

As long as Winter Orb is untapped, players can't untap more than one land during their untap steps.

Card Rulings

6/8/2016 All permanents untap during a player’s untap step at once. If Winter Orb is tapped as your untap step begins, your lands will all untap.
6/8/2016 You can’t tap Winter Orb just because you want to. Another ability, such as that of Glare of Subdual, must instruct or allow you to do so.