When people refer to 'slow twitch' and 'fast twitch' fibres, what does this actually mean. I realise that slow twitch tend to be recruited in less intense, more prolonged activity and fast twitch in more powerful, dynamic movements, but what I don't understand is the physiological principles behind the 'twitch' element.

Your initial evaluation is correct. Slow twitch muscle fibers (Type 1) are used for longer duration and less intense activities such as running long distances or lifting lighter weights. Fast twitch fibers (Type 2) are recruited for short duration, high intensity activities such as weight lifting and sprinting. There are also 4 types of fast twitch fibers categorized as Type A B C & D. A fast twitch fiber is categorized as such based on the difficulty or intensity of which it is recruited. Type A fibers are recruited first and Type D fibers last. Muscle fibers are in groups called motor units. A motor unit consists of several hundred muscle fibers and one nerve that delivers a signal to all of those fibers. It is actually the nerve that innervates the motor unit that dictates what types of fibers are in that motor unit. This is the theory of researchers that have shown that the conduction velocity of the nerves that innervate fast twitch motor units have higher conduction velocities than the nerves that innervate slow twitch muscle fibers. All of the fibers in a motor unit are of the same type. When a nerve delivers a signal to a motor unit 100% of the fibers in that motor unit contract 100%. This is known as the "all or none principle". In each muscle there are 1000's of motor units. However not all motor units fire at the same time. In fact a small percentage of a muscle is actually contracting during say, a biceps curl with a lighter weight. Most of the fibers in the biceps will actually be resting if the intensity of the contraction is low. If the weight is very heavy, potentially of the muscle fibers will be activated. The fiber type and the number of fibers being activated or fired is based on an individual's genetic make-up, the amount of weight being used (intensity), the training age of the lifter, and the chronological age of the lifter. The lifter that has been training for many years is going to be able to simulate the contraction of more motor units and reach more of the C and D motor units. Also, the chronologically older lifter who is more physiologically developed will be able to recruit more motor units. Muscle is not fully developed in most people until their early 30's. Hopefully I didn't overwhelm you with too much info there. Muscle physiology is a very complex combination of structures, and chemical reactions. You might want to look at amazon.com for a good Exercise Physiology book for more in-depth information.