If you pay close attention you can notice Immi was demonstrating a weight shift and desired retraction of the arm after the impact. In the Israel Defense Force (IDF), the idea in teaching punching or strikes was more to concentrate on a quick efficient fast reach blow to the opponent without projection - meaning without standing in a projected fighting stance.
Since Krav Maga defenses were built to counter an overall myriad of Human Capacity to deliver, kicks punches, blows and various strikes, or take down attempts, there was no use to many of the boxing drills, but instead it was chosen to concentrate on effective and efficient strikes.
Imrich is demonstrating what a punch should look like coming back to the center court after execution. He emphasizes proper arm retraction immediately after the impact. He also mentioned that later the cross punch can be delivered. You can see in the follow up lesson in the video in the above link that I elaborated on it in great details sufficient enough to help anyone comprehend. Of course if that does not work, it is your responsibility as an instructor to make it work!
They key in punching is accelerating the arm and maintaining maximum speed during the impact with the opponent chin or other areas desired. Retraction of the arm with the impact helps maintain the speed as if you strike a pause you must have slowed down before you came to a complete stop. The other part of the formula is the weight shift and is done but pivoting the shoulder and putting all the body weight possible behind it. The only retraction you execute is the arm as you do not want to idle to back to your centercourt. Instead you rather wait and see what you need to do next incase you've missed your first shot. So your follow up strikes will start from your last position you were in. Idling back slows down your focus and attention in the critical split seconds and could give your attacker the valuable time to counter attack.
It some striking methods of other martial arts, the force is done the weight shift is done with a shock like motion with tightening all the muscles in the body with the strike. Instead of returning to the center court, you then strategically evaluate what options your opponent might have next and follow up accordingly. If you missed the first strike, and you are moving to the center court you are idling which subjects you to a counter attack.
This is a fighting tactic I learned form Eli Avikzar, the Second Israel Defense Force Krav Maga Chief Instructor and as the Third IDF Krav Maga Chief Instructor, I've put this tactical approach into the training steps of the hand Strikes learning process starting with the basic breakdown of the sequential execution training steps.
In addition it facilitates a better training breakdown to the correct sequential execution required for a KO Punch. It ensures your body weight is supporting the hand during contact and penetration with the target which could be a few inches before or behind the estimated location when you start to lunge your strike.
In fast speeds the untrained eye would not notice the difference when someone is shadowboxing, but the idea is to slow down in training in order to assume control of the unseen.
I have a very pleasant memory of listening to Imrich Lichtenfeld at the time a well aged man in the 1980's talk about the details of what a Knock Out punch should be sitting in a Cafe Ugati in Natanya where occasionally he was demonstrating it and explaining it while sitting, and occasionally getting up and demonstrating it.
Imrich Lichtenfeld a man that in his youth was a boxing champion, a wrestling champion, a glamorous gymnast, but later in his life assumed the responsibility of training the Israelis soon to become soldiers, and later the Israeli soldiers in the working art of unarmed combat - Krav Maga!
Processing the information in my mind, and then in the next opportunity trying to check myself and apply the advice... or any student anywhere looking at his instructor demonstrating in class a punch and repeating the motion with or without thinking the end result.
Better be thinking. How is this motion going to stop my opponent... Listening to Instructors recommending breaking more boards or bricks, others recommend more workouts on the punching bag....
Well each training method develops some elements but not necessarily all that are required for a knock out strike.
Continuous attack the golden solution to control an opponent hoping that while attacking the window of opportunity is increased for a knockout punch sounds like a good idea. But, the window of opportunity could close immediately after the first attempt to strike and open for your opponent instead. You need to know the whole theory before you attempt to use one idea of it.
Anyway, in the IDF training steps had to be modified for the favorite KO punch in order to simplify the complexity of any human motion and bring the soldiers for a speedy control of the unarmed combat theory and application.