This is pretty cool. The Henson company has been releasing the occasional backstage video. Here we have a screen test from Labyrinth where they are looking at the characters’ relative sizes. One of the nifty things is that Ludo doesn’t have his fur on yet so you can really see the structure of the puppet.
What are the cables coming out of Ludo’s back? Power for an internal fan?
Those are likely the power to the monitors inside the puppet so that the puppeteer can see.