Thanks again for the great technical discussion judobum
In my dojo, we were taught two different forms of the double leg- one using momentum and one using leverage. The throw shown in the OP is a perfect example of the momentum variant, which we were taught was a good option when an opponent went for makikomis or foot sweeps without committing. This happened to me frequently in low level competition, where opponents didn't feel comfortable enough defending potential counters to put their full weight into their attempts.
The leverage variant was much closer to wrestling style, duck walking in (keeping the knee an inch above the mat), and waiting for the opponent to sprawl. At the same time, you grab the pants just below their inside knee. As you feel them sprawl, you push forward to place their center of gravity more directly above you (braced on your forward leg), straighten your back just enough to get their feet off the ground, and then do a mini slam pulling their legs out from under them.
Though I was taught to use this one before getting grips, I had most success with it as a counter. For example, going up against the left grip right-side uchi mata into drop seoi combo that seems to be so popular lately, I'd use my time stepping over their leg to grab a handful of pants in my left hand, then get thrown to my knees (facing them) by the seoi. Starting here simplifies everything about the double leg, giving you an established grip, knees already on the ground, and an opponent who's trying to stand back up. Best part is when instinct takes over and they try to counter your tackle with sumi gaeshi, only to give you the point because of the hold of their pantleg =P
I think the key difference between these two options is that the first should end with a roll up into kesa gatame, while the second leaves you standing.
Sorry for the clumsy description/first post, I'm not that technical, but I do agree that this is an underutilized throw in judo, and would like to see that change. Reforming competitive judo, one wall of text at a time!