From the source article used for this Wikipedia entry, an article in the Village Voice by Wayne Barret, dated July 4, 2000:
But bar tabs weren’t the only debts Harold collected. He had come a long way since the spring day more than 14 years ago when he mugged a milkman. Now the crimes he committed were part of an organized criminal enterprise. Known as the “muscle” behind the loan-sharking operation, Harold was Leo’s collection agent, recouping money that had been loaned out and was now overdue.
Most debtors would pay at the bar, slipping an envelope to Harold across the counter. In the mid to late '50s, Harold collected as much as $15,000 a week, tapping dozens of debtors. The “vig” usually began at a stifling 150 percent and rose with the passing of each week. Many people borrowed money to pay rent or foot a business expense and would pay back four or five times the amount they borrowed. There were no excuses for being late.
One afternoon, a man reluctantly entered the bar to apologize to Harold, saying that he didn’t have the money—could he have just one more week? Frowning, Harold reached under the bar, out of sight, and gripped his baseball bat. As the man before him continued pleading for an extension, Harold swung the bat, cracking him flat across the face, sending him back a few feet. “Don’t be late again,” Harold said, according to an eyewitness.
That was the gist of Harold’s job: enforce Leo’s law through threats or violence. He shoved people against walls, broke legs, smashed kneecaps, crunched noses. He gave nearby Kings County Hospital a lot of business.
“People in the neighborhood were terrified of him,” said a frequent customer at Vincent’s, who was one of Leo’s son Lewis’s best friends and whose family borrowed money from Leo.