I didn't know this, assuming it applies back through the SNES days.
I thought I'd played a good bit of Mario Kart in various games, but I didn't know about this either, unless you mean the thing that's been around for a long time (mash A or whatever repeatedly and the item will stop spinning immediately instead of later). *edit* Oh, someone else mentioned spamming the button instead of pressing it at a certain time, so this does sound different than the starting line/Lakitu speed boosts.
If there's some specific timing to get the item immediately, I didn't know that.