It’s an accessory that attaches to a PWC (Personal Watercraft) which gives the Flyboard propulsion through the air and water.
90% of the propulsion is routed through the primary nozzel that is attached to the bottom of the Flyboard.
Movement and control is accomplished by using your feet to tilt the Flyboard up, down, left, and right. You could compare it to a powered wakeboard, but much easier, safer, and intuitive.
The hand nozzles get the remaining 10% of the power and are used to ensure stabilization, similar to ski poles. In effect the PWC or “ski” acts as a pump to give you propulsion.
As quoted by the inventor and developer, Franky Zapata: “The Flyboard encompasses all my passions; the jetski, ski, snowboard and acrobatic dives.”