by seanmccoy
I think it's most useful (like a lot of deception type cards) when you can establish a pattern of play and then break that pattern.So, for instance: If I set two hellfire traps, and my opponent walks into each of them, then the third time I can just play a decoy. He'll generally avoid the decoy now (letting me block his routes, etc.) believing it to be another hellfire trap.
However, if he sets the decoy off and learns that I've been using a decoy. I can now place a decoy and a hellfire trap out. Now, he believes that I'm using Decoys occasionally, so he's not as terrified to walk into certain zones - which means he's more likely to walk into a zone with a facedown enchantment in it, which means he's more likely to walk into a hellfire trap.
If decoy is the first trap you play, you don't gain a lot of benefit from it. But if you play it after developing a pattern (let's say you put a block and reverse attack on a large creature consistently), your opponent is likely to assume that you're playing into that pattern over and over again. He's thinking he's "figured you out," whereas you're using that assumption to save mana and gain the upper hand.