In Cantonese 放水 means not using one's full strength (in a contest against an opponent) [literal] release water
For example, a strong team is winning 9-0 against a weak team. If they want to give the weak team some face, they can allow the weak team to score once before the game ended. It is a typical 放水 behavior; In some cases, you can even 放水 to the point of your opponent actually win the game
I don't think 手下留情 is the same as 放水 because 手下留情 doesn't allow your opponent to win in any case while 放水 does
What is the term equal to 放水 in Mandarin? Can it be simply 讓?