[reader5000]

I think his argument assumes the existence of pseudorandom generators which map a small amount of "true" random bits to a large amount of bits that look random to any polytime observer. The "derandomization" is that we just have to check all possible states of the seed bits which hopefully will be logarithmic in the size of the problem so you can do exhaustive checking.

[polymipisexp]

Nisan and Wigderson prove many different corollaries of their construction in their seminal 'Hardness vs Randomness' paper but their requirement for general derandomization (P = BPP) is that there's some function f computable in time 2^O(n) such that for some e > 0 for all circuits of size 2^en the correlation between f and the output of the circuit is sufficiently low (if I understand correctly).