It is a test generator generating the 1.2/50 uS open-circuit voltage wave into a high impedance test piece (or open-circuit) and delivering the 8/20 uS short-circuit current wave into a low impedance test piece (or short-circuit). 

This is a kind of black-box testing allowing the more appropriate waveform automatically to be selected even if the impedance of the test sample is unknown. 

This is in line with the classical test method of using the 1.2/50 uS waveform when the test piece is known as a high impedance to the test voltage and using the 8/20 uS waveform when the test piece is known as a low impedance, respectively.