I'm going to try something a little different with this article. It's going to be in more of a narrative form than a purely informational one like usual. I just thought you might like something a bit lighter in your Pokéreading. It still has all of the details and mechanics of the item like you are looking for, but just in the package of a little story. If you do or don't like it, please leave feedback in the comments. I'll do some more like this if you like it, and stop if you hate it. So here it goes, a story about Choice Scarf.
You walk into a dark alley. A shady looking Gligar calls you over. He looks like he is trying to sell you something, probably illegal. As you walk up to him you spot the rare candies he wants to sell you.
"Look, I'm not into letting my Pokémon get into substance abuse. Rare Candies aren't for me." You tell the street urchin, as you get ready to walk away.
"I get it, you're a purist. Training, battling. That's what you're about, am I right? I've got what you need. It's this." He begins to pull out some sort of scarf. It's tattered, dirty, and is so torn up it looks like it might have gone through a meat grinder.
You laugh. "I don't need an accessory. I'm a trainer."
"Ah, but friend, this is no accessory. It's a one of a kind Choice Scarf. Slap this on your Pokémon and he'll be quicker than a Ninjask in November. 50% quicker, to be exact." The Gligar begins to slip it back into his trenchcoat. "But I see how you are, I won't be able to sell it to you. You've made your point. You're a trainer..."
"Wait!" You cry out, thinking of how much you could use this. Your Empoleon that was powerful but too slow to do any real damage could become your greatest asset with this. Or you could put this on one of your quicker Pokémon and you could take out that Dragon Dancing Gyarados without so much as a blink. "Tell me more," you plead the peddler.
With a twinkle of victory in his eye, the Gligar continues, "I thought you might want to take a look. This little beaut can make an average Pokémon the best revenge killer in the planet. What do you do if that Tyranitar barely survives your surf and manage to KO your Starmie? No problem. Send out your Flygon with my patented Choice Scarf and BOOM, POW, THWAP, ONOMATOPOEIA! There goes the neighborhood.
"Or," he continues, "That Pokémon you disregarded because of its lackluster speed stat can now play in the big leagues. Your Heracross can outspeed most sweepers, with this. Throw Close Combat on there, because defense drops don't matter when your enemy's seeing stars before they even know what hit them."
This is all sounding too good to be true, though. "What's the catch?" you ask, knowing that there must be one. "What is it? HP loss, drop in attack?"
"Oh, nothing," he begins, "it's just that your Pokémon might suffer from a bit of, shall we say, 'move lockjaw' until he switches out."
"You're telling me I can't switch what move I'm going to use after the first turn?" You knew that there was some negative, but this seemed pretty huge.
"Aw, come on! It's not that bad. Just use a move that not many types resist. If you come up to a bind just switch out. If you really, really want to keep using that Pokémon you could even switch out and right back in and make a different choice altogether. It's that easy."
"What happens if I try to use something else?" You ask, trying to see if there is a loophole.
"Eh, it just says you can't do that and brings you back to the move selection screen until you choose the move you're locked into, use an item, or switch Pokémon. So what'll it be? It's 5000 Pokédollars."
And so, the choice is yours. Do you want it? 50% boost to speed at the cost of being stuck into a move until you switch out. It's yours if you want it.