Just from my deposit research a couple weeks ago:
These casinos have a virtual check option. This appears to be the only method for many many people to deposit (such as the 11 states, no credit cards for them). I assume this is the ach.
You fill out a check and give your banking details on it. Then you “deposit” it at the casino and the deposit shows instantly in your account.
You can deposit $500 a day and get permission for more.
Since it is so quick and easy, I can imagine players overdrawing their accounts…. or just writing bad checks. A lot of US players are just starved for play.