Watched this clip another dozen times to really understand what he is doing, and its very simple in the end.
Its just what Stuart and other Ti coaches are saying all the time. the focus is not on the low side arm thats already in the water, but on transferring the roll into a forward spear and following that spearing line for a moment.
The low side arm is more or less without any pressure on it during that focus on spear.
In fact, its the opposite of the weight on catch technique.
Result is that the start of the actual pulling force is delayed until the body is almost flat.
From there the pulling force rises pretty steeply, using mostly the rear end of the underwater arm stroke for propulsion.
The steep rise in force causes the often seen tendency of the dropped elbow elbow/broken wrist at that point.
Its a roll-weight into spear style, instead of a weight on catch feeling, where the focus is more on transferring the weight into pressure under the low arm as soon as possible.
This roll into spear/no catch pressure style works better on relative low speeds is my opinion. At higher speeds you cant afford to not keep your finger on the trigger for more continuous traction taking over from one side to the other without too much gaps.
This weight on catch style needs more postural setting up tension and forward shoulder stabilisation effort, but its worth the effort in the end for a more powerfull smooth stroke for sustainable speeds above 1.30 min/100m.
Its a bit like Dan Empfield wrote earlier.
You have to put in a certain minimal effort level to be able to reach a certain efficiency at higher speeds.