Impossible Skateboard

I saw a boy trying to ride a skateboard this evening while I was walking up at Jackson Elementary School. He was only three or four feet tall. The skateboard he was trying to ride was a bit unusual in that it only a single wheel in front and a single wheel in back. I had seen them before, but had not a chance to study them. The front and rear portions of the board can tilt independently from side to side. The single wheel is a caster, so tilting the board causes the caster to swivel and board to therefor change direction. Normal skateboards are propelled by pushing off against the ground. This board is propelled by (I assume) some trick of wiggling the feet and/or shifting your weight. The boy was struggling with this board, but he was making some progress. I saw him get the board to go about ten feet before he had to step off.