The Release system is the system that lets you throw your Pokemon out of its Pokeball and make it walk around.
Its not only for that! It will also be used in battle! Also other features from this system include a "Make Follow"
system which when you left click your pokemon when its released you can choose "Make Follow" and it will follow you around! To release a Pokemon, you can press any of the 1, 2, 3, 4, 5, 6 keys while clicking the ground. Each key corrosponds to a different pokemon in your party (e.g., you press the 3 key and your third pokemon in your party is released) Pretty nice, eh? Also if you had the same problem as me and you cant release your pokemon hit your Number lock key and try again.