Getting paid early is essentially an automatic "Payday loan" provided by whoever you bank with (whatever institution receives your payhecks) as a perk of banking with them. I personally decline this service, because I don't like adding a paycheck-sized liability to my accounting books for two days.
My bank seems to trust my paycheck deposits, though, and they "clear" (update my balance and become spendable) under 24hrs after they show up as "pending."
They're based on notification from they payroll company about the incoming paycheck. They're not just assumed by the bank to be there, and I'm not charged any interest on it, so I wouldn't necessarily lump them into the same category as "payday loans" which usually carry extremely high interest rates and often don't actually have any basis on truly knowing if the paycheck is incoming or not. Getting paid early is not similar to going to a payday loan vendor and getting a loan.
So when my bank is crediting my account with my paycheck early, its because my work told them I'm getting paid that amount. Otherwise they wouldn't necessarily know of the amount. Sure, its like some kind of loan in a way, but its essentially paperovering the slowness of the ACH to actually clear in a decent timeframe.
Direct Deposit. The banks can see the incoming transaction before the amounts clear. Payroll often starts the process two days early anyways since they try and get it to land on the 1st and 15th or whatever the dates are, so when the bank sees the incoming transaction it'll just automatically give me the funds while the clearing happens in the background.
Its honestly the same kind of logic they're using when you deposit your paycheck and it seems to clear turbo fast.
My bank seems to trust my paycheck deposits, though, and they "clear" (update my balance and become spendable) under 24hrs after they show up as "pending."