Slingshot Surefire spreader bar, if you can find one. It has a "slap" release that lets the hook itself open up. Around 100 USD is the cost. Not sure if they're still making them...(???)
You'll probably get a bunch of Surefire haters on here, but I believe them to be the quickest way to release the kite- hands down, and it gives my students more peace of mind to have two primary safety releases.
I have heard all the stories about them breaking, (one of mine broke on me after 5 1/2 years of 100% reliable
), but your kite still flags to the leash. There's definitely room for improvement, but its THE SPOT where you are connected to the kite, and the most overlooked area in safety. Maybe some innovaters around here will come up with better designs (hint, hint
As far as kite knives go, most of them are junk and the micro pouch that comes with it makes it even worse. (We need some friggin' Freddie Kreuger Ginsu gloves or something!.
There's many discussions here that offer some good alternatives to single blades.