It is not necessary to believe in tulkus,
and there is a lot of benefit that one can get from studying and practicing dharma
even if one is not really convinced of rebirth.
However, the process is explainable in a way that is quite feasible.
First of all, consciousness is not a "thing" in itself.
It is a combination of causes.
Ordinarily, when a person dies,
these causes scatter.
But with proper training and meditation,
this does not have to happen.
Consider this analogy:
Two trucks are driving along a road near a river, where there is a sharp turn in the road.
Both trucks carry a load of wooden boards.
The first truck goes around the turn too quickly, all the boards fall into the river,
wash down stream and end up randomly all over the place.
The second truck goes around the turn too quickly, all the boards also fall into the river,
but there is a difference.
These boards had previously been put together as a boat.
The boards, as a boat, still drift downstream,
but they stay together and wash up again, all in one bunch, as a boat.
basically, a tulku has been able to build his lumber of consciousness into a boat.
When entering the stream of death,
his consciousness does not totally scatter, as it would had he not trained his mind.
Just a rough analogy.
Profile Picture: "The Foaming Monk"
The Chinese characters are Fo (buddha) and Ming (bright). The image is of a student of Buddhism, who, imagining himself to be a monk, and not understanding the true meaning of the words takes the sound of the words literally. Likewise, People on web forums sometime seem to be foaming at the mouth. Original painting by P.Volker /used by permission.