Due to society's general reaction towards racism, this is an alternative spelling/pronucation. to empahsise the fact that this is intended as non-racist slang. The 'h' is there to drive home the point that we're talking about nigga rather than nigger
Where I am, this is generally pronouced with a hissing 'h' made at the roof of the mouth.
It's hard to explain. Try going to say 'nigg' and then making a sound that is a cross between 'ah' and 'ow' and something else as you curl your top lip slightly upwards. The end sound should resemble a cross between: drinking something cold (ah! as in
That's better, I was thirsty) that catches you ('ow' as in you drank it too quickly and it gave you
brain freeze) and a kind of 'I bit my tongue' noise.
I think that's the pronunaction. Of course, the accent could just **** it up a bit.
Or I wasn't thinking about it too much.