Need help with display condition for backwards-walking animation

While I'm waiting for help with the airship sound, I'm off working on a different entity for my mod! I'm like a modding roomba, hit a wall and bounce in a different direction! I always start by asking google, but none of the returns are current. 

My new ridden entity requires a animation for walking/moving backwards or in reverse. I have the animation prepared and I know I need a display condition, but cant figure out how to either detect entity/player movement, or link it to a key bind when I press "S"... I tried a couple of different checks, but they didn't work so I've done it wrong.