To test/characterize how quickly the regulator responds to changing load conditions:
Design the DIB with high-speed switches to switch in different loads. Turn-on time orders of magnitude faster than the regulator transients. Mechanical relays cannot be used as their turn on time is typically 5ms.
Connect the output of the regulator to a high-speed digitizer/sampler on the tester.
Capture the output while switching in a load. The digitizer is triggered at a known instant and then the load switch is closed. The regulator output may be unstable for some time and then settle.
The captured waveform which is post-processed to determine how long it takes for the regulator output to settle within, say 1% of its final value.