Almost any attack hard or soft can cause a split second of distraction which puts a defender in a state of temporary confusion unable to see what is going on for a split second.
The essence of a good reaction is the ability to identify the original attack as realistically reachable, and take advantage of counter attacking in the split second where the opponent is trying to reach you as you block or evade this attack.
The real problem with feinting or the first move of continuous attack combination, is not whether or not the strike or kick were fully executed, but more whether the point where they were executed was in a range facilitating a hit. It becomes more of a problem if that point is close to the border of your range.
Let’s say you try to identify the opponent's capability from a certain distance and angle and he keeps changing his body positioning as he gets closer. You should not have much of a problem in moving in and preemptively attacking because he will be in the midst of not knowing where he is.
But lets say your opponent throws a kick in the air calculating exactly which direction you would be moving to escape, deflect or counter attack, and he managed to retract the kick before you counter attacked him, and send another attack attempting to break your defense line. You could be in danger if you did not master the Pure Krav Maga Defenses vs. Kicks which include in their tactical execution an answer for this scenario as well.
The recipe for a good reaction lies in Pure Krav Maga's Techniques and Principles. Learning how to apply them is a crucial factor in this Training System.
Once you mastered the techniques and principles, you need to go through a few simple drills where the training partner in the role of attacker is trying to change his same attack or combination of attacks and try to break your line of defense in the respective position.
The defender repeatedly should attempt to succeed in his defense. In essence you do not want to be caught up in any rhythm, not your own and not your opponent's rhythm.
Your response to your opponent's change of rhythm should be only based upon the distance he is positioned from you. When he is getting close to the hot zone (the distance when he can lunge with a kick or a hand strike) you need to preemptively attack!
In Pure Krav Maga you practice defenses and counter attacks to surprise attacks, and trying to adjust your state of awareness according to your current environment.
In training, both opponents are aware and expect an encounter with a proficient opponent.
Krav Maga - Use Your Body as a Weapon!