I don't know the details but I'd guess that there's no funds verification involved just the validity of the account itself (which is what the micropayments are for when validating a checking account for the ACH flow).

Unfortuantely, creating a memo post (hold on funds) is impossible given today's ACH infrastructure. That said, you can use Plaid to monitor funds availability throughout the lifetime of the transaction, and to predict the likelihood of clearing.