I think that video is confusing and he says some things that don’t mean anything to me. Regardless, you need to get your center of gravity moving forward to build momentum and you need to lead with the front hip to set up a load for forward trunk flexion later in the delivery. Trunk flexion occurs after the shoulders rotate. It is simply the bending foward at the waist that happens about the same time that the arm comes forward. Setting up the load for trunk flexion is simply adjusting your posture so that the upper half stays back in a position where it can really “snap” forward.
You can practice leading with the front hip using a couple drills. First is the Hershiser Drill. In this drill you stand next to a chain link fence or padded wall, lift your front knee, push the hips sideways, and stride into the fence/wall. Your front hip must be the first thing to touch the fence/wall.
The second drill is the cross-over drill. You start as if you’re going to pitch from the stretch but you cross your stride foot over the pivot foot. Bend both knees a bit so that both feet are flat on the ground, Stick your front hip out towards home plate a bit if you want to exxagerate things. Then pitch from that position. The cross-over position pretty much presets you in a position where the front hip is leading.
You can also do a cross-over version of the towel drill which I would start with before throwing.