Simultaneous defence and attack is not a concept specific to Krav Maga, but it is essential to our principles and you must apply it whenever possible. The reason is simple: Efficiency.
The faster you can overwhelm your opponent by causing pain, off-balancing, and disrupting, while applying Retzef and protecting yourself from the initial attack, the more successful you will be.
Of course, it is not always possible to do both. If you were not paying attention and were in condition white, it is likely that you may only be able to defend until you can “activate” and enter into full Krav (“Fight”) Mode.
The concept of simultaneous defence and attack is the ideal scenario and can often be seen in such techniques as the “360 defence,” in which we try to both block, burst, and strike as closely together as we can.
Your efficiency will depend on your readiness level, your skill, and your overall physical response time. The more defence and attack techniques that you have trained to apply simultaneously (or close to it) the more likely you are to survive a given situation.
It should be noted, however, that, if you are applying this concept, you are in “stage four” self-defence, you are acting Reacting rather than Preempting; meaning that you failed, or were unable, to apply the previous stages.
Like many Krav concepts and principles this is a guideline; while it should be applied wherever possible, you still must be able to adapt when circumstances change. At the end of the day, what you do in a violent confrontation will be considered a success if you managed to make it to safety with minimal damage.
*Topics under any principle category (Eg. Krav Maga Principles) may be updated from time to time. So check-in every few months to see if the posts have been updated.