Had me a little puzzled as well but found a similar one on the net and I guessing it slides under a machine on it's legs, is height adjustable to suit the different eras and then lift, lifting the game with the handle roughly popping out just in front of the front of machine / coin door. Like a segway have the vertical frame and wheels located under the cabinet in the perfect Centre of Gravity point and wheel around. though can't see this used at steps, staircases etc but for a pinball show and through a pub could be ideal.

But I am totally guessing as couldn't find any pics of it being used so I could be way off.

http://user.xmission.com/~daina/client_ads/pinball_cart.html