Schlieren airgun (air only) 1/8th speed

Quick test of a tiny QEV cannon firing air at 12atm, viewed through a Schlieren setup (no razor blade). 1/8th speed (15fps) and improved video performance because somehow Youtube cropped the last frames of the other videos (?) I didn't quite understood the need for a razor blade but without one I still see the flow of air. For me, the razor blade just makes the dot of light smaller(?) If anyone can clarify me what the razor blade really does and where it should be placed (in the source of light or in the camera) I'd really appreciate. Also important to note that this is AWESOME to look with the naked eye but almost the dimmest light can blind you. I covered a tiny LED torch with aluminum foil and punctured a hole with a sewing needle, on the side! so that the LED doesn't bright directly through the hole. Take care with the eyes!!