Impede Momentum

• WOTC Official Reminder Textofficial rules
  Scry — Look at the top N cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.

• WWizards of the Coast2022-09-09
  Official Ruling
  Stun counters replace untapping for any reason, including players untapping tapped permanents during their untap steps.
• WWizards of the Coast2022-09-09
  Official Ruling
  If a tapped permanent with a stun counter on it would become untapped, a stun counter will be removed from it instead. This is a replacement effect.
• WWizards of the Coast2022-09-09
  Official Ruling
  You may target a creature that is already tapped with Impede Momentum. If the target creature is tapped as it resolves, you will still put three stun counters on it.
• WWizards of the Coast2022-09-09
  Official Ruling
  If a permanent has more than one stun counter on it, only one will be removed each time it would become untapped.
• WWizards of the Coast2022-09-09
  Official Ruling
  If untapping a permanent is part of a cost (such as that of Halo Fountain’s first ability), you may pay that cost by “untapping” a tapped permanent with a stun counter on it. The stun counter will be removed and the creature will remain tapped. However, the cost will still be paid.
• WWizards of the Coast2022-09-09
  Official Ruling
  Abilities that trigger when a permanent “becomes untapped” won’t trigger if a stun counter is removed instead.

Rulings via Scryfall