Positive Pay is basically just a reporting feature: it doesn't create any payments. The principle behind it is that when user send to the bank a list of cheques they have issued. The bank then references this list when they receive a cheque for payment to ensure that the details on the cheque match those that were printed on it when it was issued.
This is a requirement for fraud detection at most US banks and in some other countries. The PP feature basically uses a bank file template to output the information on the cheques within the specified number range.The feature is benign in that the user cannot initiate payments using it.