Thank you for bringing up the topic, James! If there ever was something I haven't been able to comprehend, it's a capping inversion. It seems that there are several days during the summer here in Northern Minnesota that are absolutely primed for severe weather activity. However, when that "magic hour" comes along, storms happen elsewhere in the region (say, within 40 miles in any direction) and nothing ever erupts here. As a result, I have seen many blue-sky busts and only a handful of decent severe weather days. I have never quite figured out why that was...
Amanda, as you know, the cap is also referred to as a 'lid'. That term aids in visualizing what is happening when there's a cap in play over moist, unstable air.
Imagine keeping a lid on a pan of boiling water. The longer the lid stays on, the harder the water boils, and the less time involved in getting the water to the boiling point (an increase in the amount of potential instability).
If you lift the lid too soon, you don't get to the boiling point as easily or as quickly (faster loss of available instability).
The longer the cap stays on, the more unstable the air beneath.
Again: the harder the boil (available instability) beneath the lid (cap), you tend to have a 'boil-over' which can be similar to what happens when a cap breaks above an area of large instability.
Caps have boundaries; strong caps can hold the energy in beneath ('boiling over' analogy again), while storms forming along the capped area, along it's boundary, can become severe (water escaping from around the lid of a pan).
Then again, if that cap it too strong (lid is tight and sealed as on a pressure cooker), no storms for you.