"We didn't use our best line-up in our playoff game"
Teams will get this info in their news after a Playoff game, in case they didn't use at least their 1st best or 2nd best line. The line evaluation happens automatically and you can check it in the game report in the lines section, column "Rat".
Also your opponent will have this item in his team news:
"The opponent didn't use the best line-up in the playoff match, our players gave up the financial reward for winning" As a result, the opponent's team will pay no winning players bonuses for this game, even though the motivation was boosted, as if the bonus has been paid normally.