We've all seen it - a villainous creature (or person with retractable claws, especially fingertip ones) holds out a paw/hand and then suddenly extends its claws - normally without any accompanying motion - in order to imply a threat or to punctuate a verbal one. A great example is in the Adventure Time episode entitled 'Dungeon', where the Demon Cat tells Finn, and I quote:
"You're trespassing in MY dungeon, manling...and I'M the thing..."
- insert claw extension in middle of dramatic pause*
"...you should be afraid of."
There's a video here: http://www.youtube.com/watch?v=W9_iQ1FSnp8
I swear, this gets used everywhere. I couldn't find a trope for it, but if there is one, please let me know.