You hold the butt of your cue too far back. At CB address, your forearm should form a 90 degree angle with the cue/floor. What you are doing is holding your cue further back, then your stroke finishes with your forearm 90 degrees to the cue.
Try this...put an OB 2 feet from the pocket, and put the CB two feet from the OB, so that you have a straight in shot. Move your butt hand closer to the joint of the cue and practice pulling the cue back slowly, pause, then gently swing through the CB so that your butt hand hits your chest on the follow through. That will turn a poke into a stroke.
Watch your opponent in the video. He starts with a good 90 forearm angle with the cue, and he follows through past that 90 degree point, unfortunately he drops his elbow so badly that his butt hand finishes below his belt. If you don't drop your elbow, your hand will naturally swing up into your chest.
There are instructors who teach that how hard you hit the cue ball should be determined solely by the length of your backswing--with nothing else about the swing changing. Personally, I find that on really slow speed shots, I have better touch if I don't hit my chest with my butt hand, but on shots in the range from the upper end of slow speed <---> fast speed, I try to swing the same and just change the length of my backswing.
