I have noticed how easy it is when training with a partner on self defense movements to lose one's serious focus, and mental attitude. There seems to be a little voice at the back of your mind that interrupts your actions with "I do not really want to break this person's leg, neck, arm, etc." Often, I've seen people become uncomfortable with the content of the self defense move, and be reduced to embarrassed giggling especially when practicing a groin kick, or a violent choke hold.
YET, to truly be effective in applying the movement in real life, one has to see themselves doing the action, and mentally agree with it's results, and consequences. We have to be willing to commit ourselves to the action. There has to be a "follow through" to the movement, even if it is only a mental imagining of the happening. Otherwise, we are practicing to stop ourselves from completing what is necessary in that kind of situation.
"At any given moment in our waking lives, our brains are flooded with a bewildering array of sensory inputs, all of which must be incorporated into a coherent perspective that's based on what stored memories already tell us is true about ourselves and the world . . . the brain . . . [sifts] through this superabundance of detail and [orders] it into a stable and internally consistent 'belief system' - a story that makes sense of the available evidence. Each time a new item of information comes in we fold it seamlessly into our preexisting worldview (Ramachandran & Blakeslee, 1998, p. 134).
The brain fits new information into existing categories or understandings based on prior memories and learning, keeping the person's worldview relatively coherent over time. This means that the brain helps "create your own 'reality' from mere fragments of information, that what you 'see' is a reliable-but not always accurate-representation of what exists in the world, that you are completely unaware of the vast majority of events going on in your brain" (Ramachandran & Blakeslee, 1998, p. 228). This may explain the individual's ability to deceive him or herself and to create and live in another reality, a reality that others have difficulty accessing or understanding." http://www.westga.edu/~distance/ojdla/fall63/meyer63.html
So therefore, when learning self defense applications your former experiences, your inner mental attitude, and the present experience will affect how well you can use the information being presented to you in the future. In other words, will you be able to avoid, or stop an attacker from harming you?
Kata/Poomse/Patterns are a wonderful tool for creating the right mental focus, and attitude as you do self defense movements. You can mentally visualize yourself breaking through the attacker's arm, or throwing the attacker to the ground and breaking his neck without actually physically doing the move to a real person. Since your brain cannot distinguish between real and imagined, it will treat these visualizations as actual experiences, and help form the right kind of reality needed if ever placed in that type of situation. The great thing about Kata is that you can solidify, and ingrain the right attitudes, and responses through repetition. The more that you take your pattern seriously, and envision what each move can do for you, the more you invest in your Martial art.