"The essence of Krav Maga is economy of motion coupled with imaginative embellishments on the other martial arts, which are too inflexible."(Imrich Lichtenfeld).
In the picture above taken from January 1967 publication of the IDF Krav Maga Manual, Imrich Lichtenfeld is demonstrating Krav Maga Front Hand Straight Hand Strike, Rear Hand Straight Hand Strike, a Hook, and an Uppercut.
Originally Imi started to do a boxing hook, and then with the help of Eli Avikzar that stretched Imrich's Principles and Ideas to surpass Imrich's boxing habits, determined that bio mechanically you get more force when the palm is facing you.
Note that in an horizontal elbow, the palm is facing down, and in some Karate hook strike to the temple hitting with the with the two knuckles back side, the palm faces the opponent.
The idea in a hook is to create an angular weapon with leverage using your arm, so you basically lock your elbow at some point before the contact with the target.
Also a hook can slip through the opponent's arms that are placed in front of his head, and an elbow would not slip, but elbow hook can generate greater force and is more preferable to use in Self Defense in the very close range.
The idea was to preserve the reasoning for hitting with the first two big knuckles which are more resilient to impact including the little bones in the wrist behind them (the wrist positioning is more stable to withstand an impact) Remember that some martial arts prefer to use the small knuckles of your fist, and some train them all to be resilient by continuous striking on dry beans, rocks, and even hot tar.
The hook is good for sparring with boxing gloves and better than getting a thick headgear and using elbows since a thick headgear tends to block your peripheral vision.
Also regarding to the front punches, you can see that Imrich ducks his chin and therefore lifts the elbow up with a straight punch so his head would be protected at the same time. But if you take Immi's principles and apply them throughout the system, you want to concentrate on natural instinctive efficient combination of all the options.
First of all it is more important to learn how to strike with enough stopping force. Distraction is good for the first split second, but lets face it you are then too close and need to have a stopping force.
Defensive body positioning is a good idea. Taking hits on the hard parts of the body, or protective muscles rather than the vital organ, arteries or cartilage is much a preferable option. Off course it is better to deflect the hits altogether whether by getting closer to the source limiting acceleration and delivery of maximum force.
But overall every impact causes a distraction. If you train to be the one that creates the defensive impact, positioning yourself tactically in an able mode to attack and distract the opponent at the same time.
Sometimes bursting and hitting the opponent off guard is preferable, and sometimes bursting while protecting your body in a deflective instinctive mode is preferable like when you execute a defense vs club headshot you duck your head not wanting to take a chance incase your attacker is swift.
But since the 360 outside defense is basically a drill taking instinctive body reaction and deflection and improving them to the best biomechanical and tactical advantage, you need also to consider your peripheral vision in any self defense scenario where you could be distracted, and not pay attention to an impact with a sharp, blunt or harmless body contact that will trigger a chain of event that will lead you to be lying dead on the floor.
I think instinctively a person might do it anyway when bombarded with strikes and also his outside instinctive defenses will instinctively be shorter and less effective. Sometimes a tucked chin causes to loss of peripheral vision especially if you are ducking under the opponent.
So it might be useful as a tactical approach if you are bursting in mitigating the effect of a blunt object or hand or foot, but you cannot afford not seeing a sharp object that very small force can get you injured bad or killed.
If you are sparring or preemptively trying to stop an attacker you should start using your kicks first if possible, or will react with an instinctive hand defense if you were caught off guard.
If you try to burst with a hand strike in a boxing match, perhaps you should keep your chin tucked. But at the same time any extra addition to your position cause a subtraction from direction of maximum force to one direction.
To facilitate more imaginative use of the techniques, one must understand all the options and not get bogged down with the "correct" method of a technique, but rather understand what each component is contributing to and promoting not, but yet work to perfect an overall efficiency and therefore effectiveness.
Pure Krav Maga Sample Punching and Kicking Video Clip!