Well I would experimint, in "Autodesk Combustion", with a long life motionblur only applied to a copy of the clip that has and inverted chroma key of the colour of the knife blade. However, the blur may need a directional force of some kind because the knives stay in the center of the frame all the time thus creating no left to right motion to blur. Then, as for the people that pass the screen between the camera and the knives, i would say that you could propably film that seperately over a blue/green screen and super impose it in later on. I myself havent tried this effect but i will try and make a tutorial video for you and post it on my website.. http://www.russell-lewis.co.nr but have patients as it is the beginning of the week and i have school at the moment. I shall post another reply on here to let you know when its done...