A super simplified example:
Let's say $100,000 is bet against the spread on Minnesota/Maryland. Of that, 70% is bet on the Gophers to cover ($70,000), 30% is bet on Maryland ($30,000).
If Maryland wins, the casino pays those winners at a ratio of 10/11. So, the winners get their original cash back ($30,000), plus 90% ($27,000), while the losers get nothing back.
In this scenario, the casino has a net of $43,000: (the $70,000 bet on the Gophers) - ($27,000, the amount paid to the winners beyond their original $30,000).
In a scenario exactly like the above but where the split is 50/50, the casino has a net of $5,000: (the $50,000 bet on the Gophers) - ($45,000, the amount paid to the winners beyond their original $50k). As long as the money split is 50/50, the casino will always take in more than they lose.