Same principle as the regular cascode, as in common emitter amplifier feeding a common base amplifier, but using a PNP instead of an NPN.
I actually built this circuit today and the real circuit performed incredibly well gain of 326 at 1Khz, -3db point greater than 1 MHz. I used a bc549c for the NPN a 2n3904 for the PNP and a bc517 for the Darlington buffer.
