Kardec and his fellow Spiritists first studied these effects when they observed phenomena such as table-turning, raps, and levitation of objects. Eventually, they formulated their own theories; however, when they inquired about the mechanisms of the phenomena to the Spirits, they found that the answer was quite different from what they supposed.

So, how are these kinds of phenomena made possible? Well, the general explanation is that for these phenomena to occur, it is necessary to have the presence of a medium of physical effects, whose perispirit projects outward a nervous-fluidic emission, and the discarnate Spirit then makes use of these animalistic fluids (also called ectoplasm) of the medium, combining them with its own magnetic fluids to acquire the force that produces the phenomena.  This explanation summarizes the answers and clarifications given by the Spirits in answer to the inquiries made by Kardec and his colleagues.  For more details on the basic principles behind this explanation, we turn to Kardec's synopsis of the Spirits' answers ("Mediums' Book" # 75-76). He writes:

"The universal fluid, in which resides the principle of life, is the chief agent of Spirit-manifestation, and this agent receives its impulse from the Spirit, whether the latter be incarnate or [discarnate].  This fluid, condensed, constitutes the perispirit or, in other words, the semi-material envelope of the Spirit.  In the state of incarnation, the perispirit is united to the matter of the body; in the erratic state, it is free.  When the Spirit is incarnated, the substance of the perispirit is more or less bound, more or less adherent, if we may be allowed the expression.  With certain persons, a sort of emanation of this fluid takes place, as the result of their organization, and it is this fact, strictly speaking, which explains the peculiar qualities of physical mediums.  The emission of this animalized fluid may be more or less abundant, its combination more or less easy and mediums will accordingly be more or less powerful.  There is nothing to guarantee the permanence of the medianimic faculty and the occasional loss of power by mediums is thus explained.

Let us here call in the aid of comparison.  When we desire to act physically upon a given point at a distance, it is our thought which determines the action, but thought cannot of itself strike the blow; it must have an intermediary which it directs, such as a stick, a projectile, a current of air, etc.  Observe, too , that thought does not act directly upon the stick, for if the stick is not touched, it will not act.  The source of thought, which is nothing else than the Spirit incarnated within us, is united to the body by the perispirit. However, the thought can no more act upon the body without the perispirit, than it can act upon the stick without the body.  Thought acts upon the perispirit because the perispirit acts upon the muscles, the muscles seize the stick, and the stick strikes the mark.  When the Spirit is not incarnated, he requires an extraneous auxiliary; this auxiliary is the vital fluid, by the aid of which he renders the object apt to obey the impulsion of his will.

When an object is set in motion, carried away, or raised into the air, the Spirit does not seize it, push it, or lift it, as we do with our hands; the Spirit, so to say, saturates it with his own fluid combined with that of the medium, and the object, being thus vivified for the moment, acts as a living being would act, with this difference, that not having a will of its own, it follows the impulsion communicated to it by the will of the Spirit."

The above process occurs whether the Spirit is moving an object, rapping on an object, or articulating sounds in the air (in which it acts upon air as it does upon an object to imitate the sounds) As we can see from the above explanation, the presence of a physical medium is a necessary factor in the occurrence of such phenomena. 

To this point, we add a few other important details:

1. Not all mediums have the same potential.  The Spirits clarify ("Mediums' Book"#74 Q.19): "That depends on the organization, and on the greater or lesser facility with which the combination of fluids can be effected; moreover, the Spirit of the medium sympathizes sometimes more, sometimes less, with the discarnate Spirits, who do, or do not, find in him the necessary fluidic power.... There are persons whose animalized fluids are, in this respect, absolutely retractile; with others, the combination is only effected by an effort of their will, while there are others again with whom the combination takes place naturally, and so easily that they are not even aware of it, and who thus serve as mediums without knowing it."

2. There are Spirits, always of little advancement, that do not understand these mechanisms, just there are men who do not understand the laws of acoustics by which they speak. This explains why they may tell you that they actually "use their own hands". The Superior Spirits explained to Kardec (#74 Q.18) that "there are Spirits so little advanced, and so material, as compared with elevated Spirits, that they still retain the illusions of terrestrial life, and imagine themselves to be acting as they did when in the body.....The effect is produced by them instinctively, without their knowing how, and yet it is really produced by an action of their will.

3. Spirits sometimes call on other Spirits to help them in producing this phenomena.  In addition, the physical phenomena are always produced by inferior (though not necessarily malevolent) Spirits, who are not yet entirely disengaged form material influences.  While it's true that Superior Spirits care not to occupy themselves with such tasks, their perispirits also lack the density of that of  inferior Spirits (this density is related directly to the degree of moral evolution), which is what acts, for them, like a "muscular" force.  Therefore, if an elevated Spirit desires to produce effects of this kind, he gets it done by Spirits qualified for that kind of work, just as on Earth, a man who works more with his mind calls upon another of greater physical strength to take care of tasks involving heavy manual labor. 

4. As indicated previously, the will of the medium (i.e. consciousness of, and desire for, the phenomena) is not always indispensable.  The Spirits explain (#74 Q.18) that the medium's will "adds to the power of the Spirits, but it is not always necessary, for a given movement may be made to take place, against, and in spite of, the medium's will, which is proof that there is a cause at work that is independent of the medium." Such occurrences that take place without the voluntary will of the medium and quite often without the medium even being aware of his contribution to the phenomena are known as spontaneous physical manifestations (see "Mediums' Book" Ch. V, Spontaneous Physical Manifestations).

5. Physical effects such as sounds and movement/transportation of objects are normally produced by Spirits of a lower order and mediums of a less developed mode of mediumistic action. Still, all mediumship has its utility and is conceded for some beneficial purpose. These types of physical phenomena serve more to awaken the dormant faith of skeptics and others of a materialistic mindset- those who must "see [or hear] to believe".  Such was the case during the beginning days of Spiritism when rappings, table-turnings and other similar effects were used to call man's attention to where he'd eventually discover the intelligent force behind the phenomena.  To a certain degree, they can be educational, but, Kardec says, "they rarely exist in connection with the more advanced means of communication, such as involuntary writing or inspirational speaking."   Different from the sounds and dazzling visuals mentioned above, the physical phenomena of the healing mediumship is executed by Spirits of a higher order and has a very noble and charitable application.

Kardec, Allan. "Theory of Physical Manifestations." The Mediums' Book. 2nd ed (1st edition FEB). Trans. Anna Blackwell (translated 1876). Ed. Livraria Espírita Allan Kardec. Brasilia-DF, Brazil:  Federação Espírita Brasileira [Brazilian Spiritist Federation}. 1986. 66-80.

Kardec, Allan. "Spontaneous Physical Manifestations." The Mediums' Book. 2nd ed (1st edition FEB). Trans. Anna Blackwell (translated 1876). Ed. Livraria Espírita Allan Kardec. Brasilia-DF, Brazil:  Federação Espírita Brasileira [Brazilian Spiritist Federation}. 1986. 81-105.