The Wikipedia article on pair-instability supernovas (PISNs) doesn't seem to give a very good explanation of what causes a PISN. My understanding of the process it describes is this: Once the core gets hot enough, it starts producing gamma rays with enough energy to produce electron-positron pairs when they interact with atomic nuclei, although these pairs will annihilate eventually, forming a sort of equilibrium between gamma rays and E/P pairs. Since radiation pressure is what keeps the core from collapsing, the fact that a portion of the radiation energy is now in the form of E/P pairs means the radiation pressure decreases, causing the core to contract. This contraction means that fusion will accelerate, and the core will heat up, creating more energetic gamma rays, which lend themselves to pair production even better. It also means that nuclei will be much closer together, meaning gamma rays will interact with them and form E/P pairs much faster, increasing the amount of energy in the form of E/P pairs compared to the amount of energy in the form of gamma rays. This means an overall reduction in radiation pressure, which causes the core to continue to collapse. This is a positive-feedback loop that inevitably leads to a runaway fusion reaction.
There are two problems with this explanation. The first is that it seems like this positive-feedback loop should start the moment fusion begins. The other is that it doesn't seem like there's a way for the runaway fusion reaction to overcome the fact that pair formation is taking away energy that could help prevent core collapse. It seems like the core should collapse directly into a black hole. Where's the gap in my understanding?