Stamina is a new feature that makes the game more realistic and simulates a pokemon's amount of energy. The meter caps out at 60. It drops by 5 depending on taken actions. Stamina greatly effects the gaming experience.
-Stamina greatly effects and is effected by Attacking. The more a pokemon attacks, the more its stamina goes bye bye. The more the stamina goes, the less powerful attacks become. This depends heavily on the pokemon. Certain pokemon are better at keeping up than others. Based on the pokemon, they will be able to keep up with consistantly powerful attacks. For example, Pokemon such as Metagross are built to endure. Metagross is a pokemon that can take advantage of its stamina. Its attacks only drop that 5 damage every 15 points on the Stamina meter. Pokemon like Sneasel, who weren't meant for endurance, lose that power every 5 points. Mediums like Vigoroth are only going to lose that power every 10.
-Stamina greatly affects and is affected by Defending. The more a pokemon defends, the more its stamina goes bye bye. This works basically the same way as attacking. Depending on the pokemon, every 5, 10, or 15 points on the Stamina meter, the more damage the pokemon takes (again, intervals of 5) through Defending.
-Stamina affects Speed and is affected by HP. HP is a major factor to speed as far as stamina is concerned. If the HP reaches 1/3, the Stamina loses 20 points right off the back and if the Stamina reaches 1/4, speed is cut in half...oh goody.
-Restoring Stamina. Restoring Stamina is as easy as calling the pokemon back to its pokeball. But this doesn't fully restore it, it only restores 10.
-Immediate Stamina Bye Bye. Basically, this only applies to Hyper Beam and Hyper Beam like attacks such as Blast Burn and Giga Impact. Such attacks immediately drop the Stamina bar to Zero and it takes one full turn to build back to where it' been before. If Stamina was at 45 when Hyper Beam was used, at the end of the next turn, it will be back to 45.
-How to tell? It's easy to tell if your pokemon is good, neutral, or terrible Stamina Wise. I'll put a +, =, or - next to their names on your character sheet.