Human beings by nature are creatures of habit. These habits are things or tendencies that are done in specific patterns which can be done consciously or unconsciously. As martial artists we call these patterns kata or a set of pre-determined movements. We drill and drill these katas so that they become "natural" or habituated. We need them to become "natural" so that our brains can use the habituated movements for pattern recognition. Pattern recognition?
When we are confronted by an opponent, we don't have any idea of what they will do. This confrontation happens in a blink of an eye and we must act. Our brains group things into patterns based on similarities. Therefore, when we are attacked, our brains act appropriately according to the recognized pattern. The circumstance of ikkyo might not be ideal but the brain picks up on any similarity in the attack that one might employ ikkyo and thus pulls out the ikkyo file to be used. Haven't you ever been in class and the teacher changes to another technique with the same attack and on the first try you do the previous technique? That is your brain's usage of pattern recognition.
Every person has a tendency. The tendency is what every cop or heist show on TV is based upon. The bad guy knows that the good guy always eats dinner at this one restaurant so they either plan to attack them or burglarize his house as he drinks the same latte at the same time everyday.
We use this strategy in martial arts as well. We train and train to get good at a technique like ikkyo so that when our opponent presents us with an ikkyo like situation we can capitalize it - we recognized that this can lead us to use ikkyo and we seize the opportunity. In order for our brains or instincts to have the "aha" moment when we recognize the pattern, we must train and not just superficially. We must train like crazy so that movement becomes "natural" or second nature. Only then can we use our habits or patterned behaviors to our advantage. No training means no habits. Having no habits means we will be left thinking about what to do when the time comes and possibly miss the boat or as Sensei was fond of saying "The moment has passed."