Stun counters replace untapping for any reason, including players untapping tapped permanents during their untap steps.

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.

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.

If a permanent has more than one stun counter on it, only one will be removed each time it would become untapped.

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.

Abilities that trigger when a permanent “becomes untapped” won’t trigger if a stun counter is removed instead.