During the interview, Takayuki Shimamura from EAD's Software Development Department revealed a prototype Wii Zapper - with a screen attached - which kick-started the development of the two-screen concept that would eventually result in the Wii U. The device also influenced the final design of the 3DS.
Here's the portion of the interview:
Iwata: That was back when we haven't yet decided on a concrete concept for Wii U. We experimented many things, and one of that was to see what would happen if there was another dedicated screen to a TV game console.
Shimamura: I thought that might come up, so I brought this as a prop today.
Eguchi: This was the start of two-screen gameplay.
Iwata: Yes. I did that, too. An experiment using the Wii Zapper controller was what inspired us to put a gyro sensor in the Nintendo 3DS system, which was in the final phases of development.
Shimamura: Yes. After development of Wii Sports Resort, Yamashita-san and I were thinking about something new that would draw upon the knowhow for Wii MotionPlus. We tested gameplay that involved moving the Wii Zapper and having images from the Wii move in sync on a monitor in your hands. It was fairly well received…
Iwata: When (Shigeru) Miyamoto-san saw that experiment, he said that he definitely wanted to put a gyro sensor in Nintendo 3DS, so even though the ship was headed out of the harbor, he called it back. This happened after the people in the hardware department had already been declared that "all features are now set!" (laughs). Even though the ship was headed out of the harbor, he called it back.
Yamashita: Yeah. (laughs) We intended to present it as a Wii U project, so when it was snatched away for the Nintendo 3DS, we were sad, but also pleased.
Shimamura: But thanks to this prototype, however, we were able to explain the structure of Wii U—having a screen in your hands—and it became more compelling.
Iwata: The way that Nintendo makes hardware is to take an idea that has arisen and make something makeshift and actually try it out.