This is merely my own idea based on the information we have from GW about souls and realspace/warp interaction.
Everything in reality is reflected in the warp. Every atom, every star. This is why the gravity of a star affects warp currents making ship translation into warp space within a star system very risky. However, the reflection is microscopic, more than microscopic, nanoscopic. Each atom reflects into the warp at an equivalent scale smaller than the quarks that make it up, so for all intents and purposes their reflections are invisible within the warp.
A soul is an interesting construct as it is a product of a much larger reflection relative to its molecular components than all other matter. This is entirely due to the electromagnetic frequencies that form the core of sentience. The energy that functions as the processing centre for living creatures' minds (irrespective of its physical structure, it could be gaseous, plasmic etc) produces a resonance with the boundaries between warp space and realspace, effectively widening the aperture of reflection.
As an organism's evolution brings it closer and closer to that point of sentience, its neural energy signature produces a correspondingly larger reflection in the warp. This also means that sentient creatures lack a 'true', bright or distinct soul in the warp until they develop sentience (often coupled with self awairness whicn in humans is around 8 months after birth). Before this its warp signature is much more like that of any other organism.
Some species or individuals within species are sentient but lack or have a very small warp reflection. This is generally because their neural pathways form in a manner that loops the resonance back in on itself effectively reflecting the warp in reality instead of the other way around. This inverted reflection means that their presence in the warp is either small, non existent or even negative which can result in other nearby souls to be disrupted and producing the results seen in the proximity of pariahs.
A psychic individual produces an electromagnetic pattern that works closer to the warp/realspace boundary, allowing their soul to act as a conduit for warp energy. Highly psychic races like the eldar not only form sentient nerual pathways at an early age (generally early in gestation) but also have a naturally more harmoneous energy production allowing for an almost perfect merging of realspace and the warp. This means that the warp energy can be manipulted with less effort and transferred much more quickly. Some humans are born with unique genetics that produce similar networks enabling them to channel more energy.
There are two things that govern the transference of energy, the degree of harmoneous resonance (the higher the degree the less 'realspace' gets in the way of the transference) and the SIZE of harmoneous resonance. The aperture of reflection increases in size through the formation of more complicated EM pathways all weaving together to create a large area of resonance that opens a wider 'path' to the soul in the warp, which in turn makes the soul brighter and bigger. The resonance of this energy lattice breaks the barrier between real and warp space, the greater harmony the more completely this is broken.
Here is a rough analogy: The 'aperture' is like that in an old camera, one of those cool iris like shutters. As the iris is constricted less light is allowed in, reflecting the 'size' of aperture. The 'harmony' is like a cloth placed over the iris. A thin gauzy material lets quite a bit of light in, whilst a sheet of canvas lets none in. Glass, although a phyisical object lets pretty much ALL the light in. these are not mutually exclusive, so you can have a wide aperture and a sheet of canvas, or a narrow aperture and a piece of glass. Some less sentient creatures manifest psychic powers, like the gyrinx or crotalid. These will generally have a high resonance (so a sheet of glass) but a low aperture (so a narrow iris, indicating a simple intelligence).
Beings such as the C'tan may have a large aperture (indicating a bewilderingly complicated EM network forming their sentience) but a sheet of canvas covering it. That is, although they are sentient the energy does not form in a harmony that matches the boundary of warp space.
Things like Warp Engines are inorganic constructions that attempt to mimic this effect. They are composed of mindbogglingly complicated energy fields held in place by magnetic and gravitic containment in an attempt to produce an energy network large enough and complicated enough to breach an area of warpspace the size of a space ship. Because of the complicatedness of the process and the energy requirements, the engine is only activated for a fraction of a second, just enough to breach reality. For this second though the ship flares in the warp with an artificial (although thoughtless) soul, attracting daemonic entities to it.
Force weapons use similar mechanisms, utilising crystals that naturally resonate with the warp/realspace boundary. When channeled by a psyker these are capable of drawing energy from the warp in an artificial manner.
Some postulations suggest that the most advanced AIs of the Golden Age approached true sentience, to the point where their nerual networks provided warp reflections and giving them true living souls. Whether this is true or not is unkown, but the technology for warp engines comes from the same time frame and where it not for their single purposed construction could be classified as an AI in their own right.
Please note this is not necessarily 'true' in GW terms, merely something I tried to piece together in a way I liked. If it contradicts with anything in the 'canon', well, you can guess which I prefer....