My understanding is that it's what used to be called a "cafeteria plan". Whatever the employee's out of pocket is for co-pays or deductibles gets submitted to the employer's human resource department and they cut the patient a check to pay the bill or pay the doctor directly.
Sometimes it takes a little longer to get the money from the patient because they won't know what they owe until they get a statement, then they submit it to the boss and have to wait for the money to pay it. I know, that's not the doctor's problem, but there is a time problem.