In the 3-life model of paticca samuppada [P.S.] this consciousness is commonly called "re-linking consciousness" (patisandhi-viññana) or "kamma-resultant consciousness" (vipaka-viññana) [see table  § ]. From ignorant actions in our past life (avijja & sankhara) it is viññana that flows on into, and arises in this new life. The idea being that viññana - in union with sperm and egg - is the definitive factor for the arising of a new life. This is conception in the full and complete sense. It seems reasonable that the process of conception and birth is clearly more than just a mechanical sperm-egg thing and, by extension, it also seems reasonable to presume rebirth. Kamma/viññana seems a reasonable medium for this but, did the Buddha intend the P.S. structure as the explanation of this? The underlying principles of condition-dependant arising can be seen within the death/rebirth process but in this context of P.S. we will consider the arising of viññana as part of the flow of this-life, of experience, in this life.

It may be helpful to explore a broader definition of viññana before finding it a position in P.S.
"When there is the sense-object, the sense-organ (in working order) and contact between the two, this three-fold union (object, organ, contact) is the arising of consciousness. If the arising is dependant on eye and form we talk of visual-consciousness (nose + odour = nasal c.; etc.)." (part Majjhima 28)
There are then six forms of consciousness - each arising dependant on one of the senses (the mind as the sixth). This a stock definition of viññana and although in P.S. the senses are not yet 'in play' this definition reinforces the dependant nature of viññana. It can not arise, nor exist independently.
"without supporting conditions, there is no arising of consciousness" (Majjhima 38)
One factor always present with viññana is intention - we have to intend to make contact with (eg. to look at) something before viññana can arise. In the frame of P.S. this intention is sankhara but the consciousness that arises here is still quite nascent, it is 'simple.' A term in Pali worth consideration is bhavanga-citta. Bhava = becoming; anga = factor; citta=mind. It is a kind of sub-consciousness that, as yet, has no (formed) subject, has no (formed) identity, no (formed) "I" within it. I repeat the word 'formed' as this consciousness does have the seeds of formation - this is sankara, the forming agent. Because ignorance was the primary conditioning factor it is inevitable that consciousness takes on, or forms, ego-identity. The "falling out of the tree" image in the introduction is clear: you will likely hit the ground.
It is this link - the arising of conditioned consciousness - that is indeed the point of birth. Not as a fetus, but birth as a seperate ego-self, a 'me' - an 'I' - a not-you. We can talk here of self-consciousness. A simple example: someone points a camera at you and you become self-concious. You may have been day-dreaming but now there is viññana - and this consciousness (your thoughts) turn to, in fact create, nama-rupa. Is your hair (rupa) tidy? You are annoyed (nama) at the intrusion, etc. There is (minor?) suffering as a result.

Consciousness can be free of the self bit - in fact this is the whole point of the teaching - but...
• with ignorance as condition > formation-intention > (self)consciousness > nama-rupa .... dukkha.
We will look more closely at nama-rupa on the next page but briefly it translates as name and form or, mind and matter. Again, this is not the body-mind development of a fetus but it is the arising in (or arising with) consciousness, of body-mind awareness. You are sitting here reading and someone calls out to you: "Hey, you've got a bogey on your nose." The nose was there all the time but suddenly it arises large in consciousness (as rupa) along with the embarassment, anger, etc. (as nama) - nama-rupa is "born."

Another way of considering the time relationship of sankhara and viññana, positioned somewhere between sankhara in a past-life and sankhara as part of a moment of this-life, is to think of your intentions (sankhara, in this-life) but say a few weeks ago. Those intentions, that kamma, perhaps took you to a new town and this gave rise to quite specific consciousness which could not have arisen in the old town. Past action results in present birth.

The theories can roll on endlessly but, for me, the main criteria in study is: What can be done with this? How can this particular teaching be put to use in my life?
It is possible to observe the subtle movements of the mind, although perhaps not every branch of "that tree" will be visible as we fall. Study is like gathering clues, collecting maps. The suggestion is that there is this intention > consciousness movement that takes place, and that it leads to suffering (when founded in avijja). Can we see it? Must we always be a victim of our (ignorant, habitual) impulses? Nah !
Break the chains of bondage - take charge of this mind. There is no spoon :)

Viññana is one of the five khandhas - see further discussion there [ § ].
It is also one of the 4 nutriments (ahara):
• material food
• (sensorial and mental) impression (phassa)
• mental volition (mano-sancetana)
• consciousness (viññana)
The details of this list are interesting in terms of what feeds life but enough here to note that it represents four links from P.S. - nama, phassa, sankhara, viññana. You will find many terms repeating, in different contexts and in a variety of (dependant) combinations, as you persue your studies. These lists all have dukkha (the release from) as their underlying target and together they build a network of definitions, a complex system of feed-back loops. Each list - P.S. included - has its specific implications and each enriches the other. Don't get caught expecting to find 'the list' - the singular, definitive solution to the tangled web that is dukkha.