Here's how Nobuhiro Takahashi and the University of Electro-Communications describe this project: "'SHIRI' is a buttocks humanoid robot that expresses various emotions with organic movement of the artificial muscles." It's designed to respond to slaps, caresses, and finger-pokes. It is super weird.
The artificial posterior is comprised of a silicon skin and a "rigid urethane skeleton," in a full-scale size. Mostly it looks like each "cheek" is a little red airbag, with modulating pressure that gives it its reaction. There's a microphone embedded to detect pressure--not sure how that works, exactly, but it can certainly differentiate between different kinds of pressure. A poke, slap, and caress were all demonstrated, and the actuators change the pressure of the cheeks to respond in kind to each one (including, like, quivering in fear).
It may not be as elegant as the string quintet robots or Herb the robotic butler, but Shiri (Japanese for "buttocks," obviously) serves the highest purpose of all: starring in a weird YouTube video to amuse us for a few minutes.