Freelensing
Detaching a lens from the camera and holding it in front of the mount, tilting it to create selective focus and light leaks.
In depth
Holding the lens away from the mount produces an improvised tilt, swinging the plane of focus, while gaps around it admit stray light that streaks across the frame. It exposes the sensor to dust and demands manual exposure with the lens's aperture set before removal, so it is a technique with real costs attached.
Examples in practice
A lens held slightly tilted renders a narrow diagonal band of a portrait sharp while everything else dissolves.