That’s a pretty complex question, you’re basically asking how the whole nature of magic works and without spending days of my life writing a 20,000 word dissertation on the subject, its not easy to explain. But what I will do is give you a very simple analogy as to how I understand magic to work. Magic is the actualisation of intent. When you cast a spell, you send energy and intent into the world and the energy you send changes the circumstances toward which you are directing your energy. It isn’t a linear affect - basic spell work is sadly not as simple as visualising a simple outcome and working to change it. Think of it more like throwing a stone into the middle of a pond. You are directing your force and will with the energy of the impact but you can’t necessarily predict exactly what affect the impact will have on the pond, i.e. the patterns it will form, the distance to which the ripples will travel… With magic, you are sending your energy into the world (some witches call it the ether or the web of wyrd) and similar to the stone in the pond, it's hard to direct the energy toward a highly specific outcome (you cannot always be sure how a spell will work). So - spells influence by channelling energy in the direction you want your intent to go but they don’t work within the confines of a simple cause/affect framework. Generally the more accomplished the witch, the more precisely she can channel her energies and this is when a witch can become more confident that she will achieve what she wants to with her spell work. However, as any witch recognises, magic has a mind of its own and quite often the results of a spell will actualise themselves in a way that the caster never imagined when creating the magic…
To use another example, think of a domino rally. The positive intentions of the witch hit the first domino and then every domino in turn has a positive impact on the next one in the line. This is a bit simplistic as domino rallies tend to work in one direction whereas magic can work in any and all directions. But you get the general idea, the intent of the witch makes changes that spread out into the world, each change making other changes as the energy works toward its target. Think of it if you like as belief and intent bending the very fabric of life, the more powerful the intent, the more potent the energy, the more change it can bring about.
There's an interesting article on Witchvox that goes deeper into the psychology of magic. I've linked it here for those who are interested in discussing the subject further.
Image http://www.deviantart.com/art/Love-Spells-Anyone-27498215 (deadgerl)