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).