'Voodoo flossing' is a mobilisation technique in which a band (usually a theraband or broken strong band) is wrapped tightly around a sore joint or muscle to increase range more comfortably.
This technique can seem to work like 'magic', hence, 'Voodoo flossing'.
Voodoo flossing works by providing a strong novel input that distracts from noxious inputs (interpreted as threatening) that contribute to a pain experience. Very simplistically, instead of feeling pain, you feel deep pressure making loading this tissue more comfortable. The extra resistance means that tissues need to stretch further to achieve the same range as pre-band. Often we have an expected range that we are psychologically driven to reach, despite extra resistance. When the band is removed, you will usually exceed the range you achieved with the band. I compare it to the feeling of walking up a hill with a heavy backpack and then taking it off to look around.
The effect of voodoo flossing is only short-term and works best when a particular area (which can be wrapped) is sensitised. I would use this technique with caution over an acute injury as sensitivity to stretching can be protective, and voodoo flossing may increase the risk of further damage to injured tissue. You also must be careful not to obstruct the blood supply to a limb completely.
Voodoo flossing isn't magic and won't get you out of doing challenging, goal-specific, responsive and consistent flexibility training; however it will help reduce the sensitivity of an area to allow a greater tolerance to stretch in the short term.