The metal conducts chakra. Asuma's chakra is wind-natured, so when he uses them, it has a different effect than when I would.

[He gestures to the blade.] Kage Mane Shuriken no Jutsu. Now that it's successful, this would be the part where I'd take your food to make sure you can't replenish your chakra.

[Which. He won't do just yet, because this was only a spar. He's curious to see if Luffy surrenders.]