That's a good example of what Coach Zog is always preaching about leg locks:
1) Securing the position of control first is PARAMOUNT. You MUST secure the position or you risk losing the position, at best, or being countered.
2) Attack from a position that leaves you with as many options as possible while leaving your opponent with as few as possible.
I dunno who the fella on the bottom was, but he missed out on that wisdom. He should have tucked that left leg in and established X-guard or at least had the left butterfly in first, then finished the reaper to a secure position; e.g. wedge or leg knot. Once you have one of those positions, your opponent's options to counter have diminished substantially. He was in a half-assed 50/50 from his back (sorry to offend the 50/50 guys -- I know, he didn't have that properly either.) In that position, his opponent had all of the control. By sticking his right leg through for the reaper without establishing his butterfly or x-guard first he gave away one of his best options to defend the heel hook; or rather, he gave his opponent an opportunity at a heel hook that wasn't there until he gave up position. He put himself in a position where his opponent had an excellent chance of countering his attack because he went for the submission without securing a position of control. Cost him a knee.
Now everybody that was horrified by that vid, go back and watch Coach's reaper series again and soak in the knowledge.