Pinching is just using some force to deshape the material of the sleeve. A lot of tension will stay in the material which can act like a spring. See it as bending a wire in a 90 degree angle but letting it go it put itself back resulting into a bend of an 80 degree angle.
A loose pinch can result into a total recover of the old shape so some pinch is needed to push out some of the created tension (the spring effect) to get a steady left over pinch.