Kite Killers are a safety system for 4 line kites.
Many kite manufacturers make and supply kite killers with their kites and can be bought seperately.
They are velcro straps which you place around your wrists and then attach the flex to the brakes lines of your kite.

Kite killers allow you to let go of your kite handles are over powered or in difficulty and still have relative control over the kite.
When you let go of the handles, the kite killers pull the brake lines on the kite and the kite will land safetly. This means if you are on a busy beach and you let go there is no risk of the kite hitting anyone or going in to the water.