There are certain things which all Krav Maga systems/styles have in common, apart from the principles and concepts that define a system as being Krav Maga in the first place. There are several common techniques which are seen across all Krav Maga systems; one of these is the 360 Defense.
The 360 Defense is a means of defending any circular attack (coming overhead, from the sides or upwards) , such as a swinging punch or a circular knife slash - remember you respond and block movement rather than trying to alter your defense based on the nature of the attack/assault. In real life it is almost impossible to recognize, at least initially, if a knife is being used; all that is obvious is that something is coming towards you incredibly fast. About 10-12 years ago someone tryied to shank me with a 9 inch Kebab Knife. what saved me was not that I immediately identified that a knife was involved and made an appropriate defense, rather that I instinctively pulled my hips back and threw my arms forward to block i.e. performed a scrappy but adequate 360 - I still have the scar from where the knife cut my blocking arm, which is a great reminder to train your defenses with your attacker recoiling the knife. Apart from having to throw away a good suit, back in the day when I owned good suits, my hand and body defense were sufficient.
This type of automatic flinch response demonstrates how Krav Maga uses the body's natural movements, reflex responses and behaviors as the foundation for its defense(s). This is one of the underlying principles of the system and one which makes it an extremely realistic and effective method of self-defense. There are many things we may like to think we would do in a situation however Krav Maga is about what we will do e.g. somebody chokes/strangles you, your hands will naturally go to try and free the choke - you won't be attacking the person but the attack itself.
Using the body's natural defenses makes complete sense when designing a self-defense system to be used by the ordinary man/woman in the street as opposed to trained athletes (or as in the case of 1940's Israel getting an untrained raw recruit competent in hand-to-hand combat, in the shortest possible time, before having going to the front-line to defend his/her country). Rather than retraining the body to work in an unfamiliar way Krav Maga accepts how we will act when faced with violence and then works with these responses to make effective defenses that immediately flow into powerful and committed attack(s).
The 360 block, which is based on the body's flinch mechanism i.e. how we respond to movements that are picked up by our peripheral vision, also demonstrates the idea of reusing techniques; another fundamental Krav Maga principle. Instead of coming up with a multitude of different defenses for different types of attack, Imi Lichtenfeld looked to see how a defense against a punch, could be re-used against a knife etc. If the body responds/reacts to movement, but is unable to initially identify the nature of the attack, then the block has to be good for both knife and open hand/fist. If two seperate defense are needed then the person being attacked will have to undergo some form of threat recognition before they make a defense. Such a process will take time and involve conscious processing - it won't be reflexive and will therefore be slower. By allowing one defense to be able to deal with two different types of attack eliminates the need to make a conscious identification of the threat and then select an appropriate technique.
It is important to always evaluate what you are being taught from the perspective of how you will behave in reality; when you are surprised, scared and your adrenaline is running high. We can all pull off spectacular techniques in a clinical or controlled setting but on the street we're left with our natural instincts. Krav Maga is designed to build on these not replace them.