This page explains what are skills and how field skills and buddy skills work.
How to switch an active character to its buddy
Players can switch an active character to its buddy while in a battle by swiping a character icon down. There is no limit for switching and players can move a character between rows as many as they want.
Moving a character between rows helps players to select a character which has an element strong against enemies. Additionally, players can use Ninjutsus continuously since a back row character also charge chakra gauge.