David, I'm not sure that I used the word "contract" myself. It is appropriate in its broadest sense, but the words I use more often are "agreement," "permission," or "deal." When I work with someone, and a spirit is present, then I know a contact has been made. The question becomes, "What is it about the client that resulted in the acceptance of the spirit's contact? Like with ET's, an ego-state of a client from a past life may be approached by the spirit of her husband from that life who did not go to the Light after he died. When that ego-state accepts the contact, I term this a "permission." The ego-state is saying "yes" to the contact. A common scenario is a past life ego-state, created as a person approaches death, feeling terrified, and is promised immortality for allowing access. It is also possible, for example, that sophisticated dark souls can insert an etheric or energy device within the soul which can travel with the soul into the Light after the person died. That device then can act as a beacon/access in a future lifetime. As Gerod pointed out, dark souls cannot follow a soul into the Light and know when and where it will next incarnate. They can develop strategies, deceits, devices, however, that, in a sense, is "marked" and can be checked for when a soul does incarnate. (Keep in mind that talking in terms of "lifetimes" is from our human perspective. Dark souls, operating in a timeless dimension, are not necessarily operating in that framework.) The possible scenarios for these contacts is endless. In its most basic terms, an agreement or permission is anytime a person, or part of a person, says, "yes" to a spirit. I'll stop for now.
(As I wrote this response, I thought I would add something else Gerod said to me early on. He said a soul can only incarnate through the Light. Souls in darkness cannot incarnate. If true, it would be extremely significant for our understanding.)