This is because your body gets all its instructions directly from your subconscious mind - the control center of all movement. So to master a new technique, all you have to do is 'program' it into your subconscious, and continue to practice the technique in the pool until have it perfected - and the results may arrive much quicker and easier than you ever imagined! The easiest way to do this is using regular visualization. This allows you to 'tape' a completely new program over the old one, just like using software for your computer - automatically erasing it and leaving the new one in its place. You do this by quieting your mind, closing your eyes and mentally imagining yourself already using the new technique you desire (as if this was the way you have always swum). However, this must be done regularly if you wish to replace the old program already running in your subconscious. It's important NOT to visualize the old technique at all, as this reinforces the current program and makes it more difficult to change to a new one. (If you accidentally do this, simply put a mental 'cross' through the image, and start over again).
Do this particular visualization at least five minutes a day whilst continuing your normal daily training - until you feel the technique 'click' into place, and you'll know you have mastered it. Every swimmer being different, there are different paths to success as well. Some will experience immediate and very rapid improvement, whereas others will experience slow improvement, but later followed by a very sudden (and unexpected) mega-successful training session.
My advice to the swimmer who asked about utilizing more leg action was this: He should visualize his arms, legs and entire body coordinated perfectly as he swam, as if this was completely natural to him. If possible, he should also mentally experience 'feeling' the sensation of the swim as well, but if he found this difficult, 'seeing' it would be just fine. The 'feeling' may come later.
Needless to say, he could not expect results unless he continued his daily physical training as well - visualizing alone will not bring results! Success comes through using both your mental AND physical capabilities. The main reason why this visualization technique works is because: We cannot achieve ANYTHING that we cannot first IMAGINE.
This Law of the mind doesn't just apply to swimming, but also to life as well. Visualize what you WANT (not what you don't) - and this will implant a new 'blueprint' into your subconscious for your body to follow. This is how it works - some call it 'accelerated learning', or cause and effect, but whatever you call it, it's light years ahead of the standard ways of learning! Use it and enjoy it.
The Mind controls the body, and the mind is unlimited. The best of success,