To me it's like a quiz style of interview, where you're up on a whiteboard trying to answer a series of rapid fire questions. There is little discussion about why you're doing what you did, only a pass/fail signified by a nod or no-nod.

In contrast, a discussion style feels more like they're trying to figure out how you approach some problem, nudge you toward a solution if you're stuck (everyone usually gets stuck somewhere, especially if they have interview anxiety. I know I do), and discuss with you why you did what you did, discuss the pro/cons of some approaches, discuss alternatives, and so on.

The first style is just checking if you know the answers to some list of things, while the second style is more like trying to get to know you as a person and as an engineer.

The drawback of the second style is that it's more time consuming for both parties. Quite tiring as well. However, I think it's part of respecting your time and respecting you as an applicant.