Percentage effectiveness, performance factor, motivation, Voodoo are just terms used to explain the fluctuations of performance......that hidden something that makes 2 identical players perform differently; that turns your team from a 14 goal machine to a whimpering wreck firing blanks; that sees your superstar striker unable to kick a coos arse; that sees your 1* DC bang in 3 from the halfway line; etc
In other words the invisible pop-up that says your team is now playing at 110%, striker has a factor of 0.8....the bonus/penalties, nerf/boosts, buff/debuff, random variability, modifiable factors that are the foundation of games.