The bassist in your photo is using a technique popularized by jazz bassist Victor Wooten about 25 years ago. (I was a friend of his back in the day.) Wooten uses one or more of an ordinary and cheap girl's hair-tie: an elastic and yarn hair-band, or "scrunchie", of the kind that can be found for 50 cents in any drug store or department store. He got the idea from his brother, guitarist Regi Wooten, to whom Victor credits most of his playing techniques.
The idea would be to roll the hair-tie up above the nut when one needs to play open strings, and to roll it down below the nut when one wants to use two-handed tapping or other techniques, to damp the unwanted sounds of the open strings when one does pull-offs.
For certain songs, Victor Wooten also uses a single, thin hair-tie in a precise position high on the fretboard to create unusual pinch harmonics in conjunction with distortion.
The use of the "scrunchie" has been widely discussed in many discussion forums, and there are videos by Victor Wooten and others demonstrating the techniques.
Google search on "victor wooten hair tie".
The Gruv Gear FretWrap, a product for bassists referred to by others on this page, is nothing other than an expensive commercial product designed to capitalize on the trend popularized by Victor Wooten of using cheap, disposable hair ties.