I don't think Otto comes online very often, so I'll try to expand on what people have said.
Regression can induce some primitive defenses in otherwise normal people, but I think what we most often see is regression in people who have a fragile BPO. On good days, they can seem fine, but under stress, they manifest splitting, projection, denial, etc, and we see them after they've cut or od'ed. We're then surprised that they heal up rather nicely, often within hours, and what they then need is often a good d/c plan rather than an admission. Neurotic/normal people may regress into splitting or projection, but it feels uncharacteristic to them and tends to be evanescent and often the response to someone who truly has a BPO--it's a fundamental aspect of becoming a psychiatrist, by the way, becoming adept at recognizing your own reactions to these difficult people.
The scaffold is a good analogy. The neurotic (ie, the rest of us) have a scaffold that affords a certain basic level of trust and an ability to step aside from emotion and look at the world through disinterested eyes. Someone with BPO has a scaffold that is intrinsically flawed in that the world is essentially seen from the primitive vantage point. They can scoop healthy stuff on top, but under stress, the scaffold re-emerges. And that leads to the central Kernberg/Kohut dilemma in that Kernberg would assert that therapy must actively confront the borderline scaffold else the alliance is built on an unhealthy framework, and Kohut would say the problem is not the scaffolding but the lack of a scaffolding, a deficit, that needs relentless addressing via an empathic treatment (ie, mirroring before interpretation).
Some of the theorizing can get a little muddled, but it's a bit like talking to an astronomer about black holes. They can present it in a sentence or paragraph quite clearly, but the reality of the situation is complicated enough that anything more detailed is inevitably going to sound confusing to non-experts. But as with black holes, there is a reality to BPO even if it's not fully delineated or understood.