In a FinFET structure, gates enclose the protruding drain and source to effectively block current leakage.  In a planar structure, the gate is attached to the transistor on only one surface, whereas in a FinFET structure, the gate is attached on three sides of the channel, allowing better control of the current leakage than planar.  Electrons move from the source to the drain through a surface under the gate in a planar structure, whereas in a FinFET structure, electrons move across the three surfaces of the fin-shaped 3D structure.  Furthermore, shorter gate length means electrons move a shorter distance for the path from source to drain, enabling transistors to switch on/off very quickly.  For simplicity, if the channel is a road, more channels means more road lanes but at shorter length due to the advanced process node.  With more paths that are shorter in length, more electrons can move faster through the channel resulting in enhanced performance.