A DL recruits the legs very strongly. When the weigh is held below the waist (DB or Bar..or even trap Bar giving a more natural grip, with the thumbs facing forward) the term Dl is used, but the legs are still involved mainly. When a bar is placed on the shoulders (well above the waist..the center point) than it is referred to as a squat, where the legs and lower to middle back become involved . BB Hack squats could be labeled a behind the back DL also.
In a true DL, in the classic sense of the word, the legs do the majority of the effort while the back structure keeps the upper body upright. There is no pull from the arms. so a DL is a push (from the legs) and not a pull from the floor. Which also shows the benefit of just holding a weigh in a static position. Holding a bar, as the legs push off the floor, can build a huge back structure. Actually only holding a extra heavy bar taken from a PC, rack, "A" frame, etc can also. build a very good back. Much to be said for static holds, which can be used by serious benchers, DL'ers and squaters. This all builds ligament and tendon strength, helping to add a thicker muscle structure.
The gentleman in the video gives a good example of the common practice of using the lower/upper back with his basic legs/hips push drive.. I'd give him 3 green lights.
Good Luck.
