Like FIFA, PES 2016 uses a chemistry system to show just how well certain squad members play together. This is called Team Spirit, and is indicated by a numerical value out of 100, present on both the Game Plan screen and the Master League home screen.
Team Instructions are the orders you give your squad on how to play, and are found in the Game Plan menu under 'Preset Tactics'. While there you'll find options which define what dreadful bores might call 'philosophy': short or long passing, fixed formation vs fluid, etc.
Back to TopTeam selection: The most obvious cause of fluctuation is team rotation: more often than not you'll see an instant change in rating just by replacing one squad member with another. This can have a big effect on your Team Spirit rating: losing a star player will always dampen moods. Be sure to try and work around this by altering the team to suit the replacement, and not the lost talisman. However, swapping a player for one whom plays in the same position similar attributes will see the Team Spirit stat unaffected.
Formations: Balancing your selection isn't just a funny-sounding Louis van Gaal line, but vital to winning games in PES. Cramming all the 'best' players you have into a team is possible in PES, but you'll have to have a formation to match their talents, which in and of itself can present a tactical downside. Changing your formation will render an instant change in Team Spirit, so if you're set on a marauding 4-2-4 then make sure you swap in players that will get the best out of it.
Tactics: Changing the way your team plays sounds easy, but in PES 2016 it takes time to get everyone playing the the way you want. Changing your passing, pressing, attack and defense mentality, or even your areas of attack will cause fluctuations in Team Spirit: huge changes can see a 90+ spirit decline to the low 80s, and even less if you then have to make changes due to injury or necessity
Back to TopFrequently playing with a set of instructions will see players get used to it, sending Team Spirit up. However, changing your tactics and not using them again for a while may see them unlearn all this hard work.