"GEMBlades use a top secret and newly developed Gravitron Electromagnetic Technology (GEM) to hover about a foot above any surface. This means that anywhere is a potential racecourse." In other words, a GEMBlade is a hoverboard.

They need keys to turn them on, like a car. The key is like a clear rod with a set of prongs on both ends. It fits in the crystal in the middle of the board, where there is a hole.

The GEMBlade floats about a foot off of the surface, and the space between the ground and the board is lit up by a haze, as though you were looking through water of the same colour of the board (each GEMBlade has a diffeent colour). When they move forward, they leave a streak of colour behind them, that lasts a few seconds.  It's just the electrons becoming excited by the graviton pulse.