When foiling, you use a foil – or to be more accurate a hydrofoil – that is attached to a board and extends into the water. Due to its shape, it acts like the wing of an airplane and lifts out the board of the water when having enough speed. Once the board is lifted out of the water, only the foil and the mast that connects the board with the foil remains in the water, leading to reduced drag force which allows to ride at much higher speeds than with riding the board on the water surface.