This card is great. 1 effective resource to shut down most minions, for an aspect that needed more options against minions. It sometimes helps if the hero has retaliate so that the enemy kills itself over a few rounds (e.g. if it has scheme more than 0 or attack more than 2), but still good without retaliate as a super cheap answer to minions.
Attach to a minion. Max 1 per minion.
Attached minion gets -2 ATK.
Okay, I know this is a card with a niche. You have to be playing Protection and you need to take on a villain with some beefy minions.
Don't get me wrong, Pinned Down can effectively nerf a minion for a round without costing additional resources. But to make the most of it you want to drop it on a minion that swings for 2 or more. But in a game where card draw is king, you don't want to hold this in your hand waiting for a moment.
Instead of throwing your solid allies at some nuisance with retaliate, you can drop a Pinned Down and ignore it for a round or two until you have what you need to eliminate it through other means.