2) How can the hick task be culturally biased?
There are half a dozen ways. Here's one.
From your description, the faster you move your finger from the button, the "smarter" you are, and the faster yo move it to the lightbulbs after you moved it from the button, the "more agile" you are.
But who says that's true?
Surely, it could be a cultural difference between whites and blacks that whites in general have more confidence or self-esteem, for instance, which makes them more likely to have their finger leave the "home" bulb even when they are not 100% sure of the answer, confident they'll figure out the answer presently. On the other hand, black test-takers might be more likely to hesitate, trying to make sure they KNOW the right bulb to move to before leaving "home". This would also explain why whites on average take longer to touch the right bulb once they leave "home" than blacks--they're still doing a calculation in their head, while the black person already finished it with their finger on the button.
In fact, it is easy to imagine a few reasons for just such a "bias". Perhaps, for example, it is precisely because blacks are more concerned to prove they are smart to the testers, in order to dispel the myth that blacks have a lower IQ, that will make them leave their hands on the button for that extra half second of "making sure". On the other hand, whites--likely to care less about what general impression they are sending about the "intelligence" of the "white race"--could take a chance once in a while, and do the test more freely, which would make them leave the button earlier, at the "price" of a longer time to reach the right bulb on average.
Of course, this doesn't prove this IS necessarily the actual bias working here. But it certainly at least a REASONABLE way in which the test can be biased--as opposed to the claims that such a bias is "impossible". By the way, I can think, off the top of my head, of at least another unrelated ways that such a systematic bias against blacks could be introduced into this test.