Genetically domesticated foxes arise out of 50 years of selective breeding. This is from the tame stock, where animals' early reactions to handlers are observed as being non-aggressive. These animals are allowed to breed and the offspring are further selected for tameness. There is no behavioral modification like petting or striking in the fox's life. See the above link for the complete testing procedure.
This video is a simple behavioral evaluation which determines how the fox responds to the handler. First, the handler simply approaches the cage. Then, the handler idles outside the cage. The cage is then opened but the handler does not seek contact with the fox. Then, the handler seeks to touch the fox. The cage is then closed and the handler stands outside the cage once again. Each phase of testing lasts for one minute (this is a truncated video). The fox's reactions are objectively recorded by the handler.
This particular fox is from the tame stock.
These videos show the impact of genetics on behavior. (See companion video of fox that has arisen out of the aggressive stock.) The "bite" you see in this video is non-forceful and a form of play. Foxes use mouth-holding to show dominance.
Rats and I believe a form of stoat were also experimented with in this way, but I do not have any videos.
Edit 4.16.07: I just learned that Belyaev found out that the "tameness trait" was actually an adrenal response. Within 10 generations of selecting for tameness, foxes had much lower adrenaline levels than their wild counterparts. The neural formation pathway for adrenaline response is linked to a host of other traits, all of which he was now observing after those 10 generations: mottled or white fur color (melonin), droopy or small ears, shortened muzzles, shorter tails. Adrenaline was only one hormone in a collection of many that ended up being effected.
On today's exotic pet market, it is possible to find a red fox that is not red at all, but white. These are not arctic foxes but red foxes with white fur. Belyaev's findings suggest that these foxes have been bred through at least 10 generations of tameness selection. However, this does not mean that these foxes are tame on the same level that dogs are. Foxes make rambunctious pets which can be difficult to handle once they reach maturity. They tend to mark food sites and posessions with malodourous urine and they have a predisposition to dig and chew furniture. (Such foxes are different from the one you see in this video, which has had its genetics gone through at least 30 generations of tameness selection)