Stacked Limiting when Mastering
Stacked limiting is limiting an audio signal and then feeding that signal into another, and possibly a further limiter if you desire.
Back in the day (around 1937), it would be highly unlikely that you would think of doing this being as the limiter had just been invented and was the size of a cupboard. It would continue to be the norm for many years to come mainly due to cost and availability but with the advent of more affordable units and of course Plug Ins, this practice has become more and more popular.
So why use stacked limiting? Thankfully the loudness wars are a fading memory for most but limiting is still an important part of the mastering stage. The technique for minimising unwanted effects is by using more than one limiter in series and not being as aggressive with the threshold knob. This works by the fact that the limiter is only reducing the loudest peaks at any one time as a significant effect on the overall sound. This demonstration is an extreme example to highlight the effect.
You may of course desire some liliting artifacts as part of your sonic intentions but you can still use this technique even after apllying this if you just want a small overall gain level without ruining your sound.