It says that doubtful occasions (shots, fouls, divided balls, etc.) will be favorable for your rival.
There isn't anything to substantiate such a statement, not to mention that "doubtful occasions" is an unclear concept. Less effort will typically cause your team to perform worse in a game, but that's about all that's known for sure.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."