Testing with bursts is only useful when using a resistive dummy load and using an oscilloscope. It doesn't load the amp too severe as well as the dummy load. Bursts of 1 kHz show general clipping behaviour, at 10 kHz memory effects (due to saturation of transistors) are more visible and eventually oscillatory recovery. Burst at 100Hz repeated at 1Hz around clipping show how sturdy the PSU behaves. It says something about bass response. A weak PSU will result into a more sloppy bass most time. Or more accurately, the power hungry bass will modulate the clipping level more.
[EDIT]
Of course this way is not a real time clipping indicator but part of characterising an amplifier. As mentioned earlier clipping also depends on speaker load: Will it clip due to running out of voltage headroom or is it current limiting? Or both, either depending on frequency. A good real clipping indicator is more complicated. What happens at clipping, if it's either voltage or current, is that the feedback loop falls apart and parts of the amplifier will saturate. So a more realistic real time indicator will detect somewhere in the amp when things saturate or being over-driven. That makes it more complicated and raises the cost.