Pin-less debit comes from online debit transactions that are processed via EFT networks ie STAR, PULSE etc.
In traditional retail if the merchant is maximizing the low cost debit networks, the consumer buys a product in the store and enters their pin number. The merchant is charged a flat fee. If the consumer does not enter their pin number and they sign, the merchant pays a percentage of the transaction.
Pin-less debit is the online solution to the retail pin-entered transaction. The merchant gets the benefit of the flat rate transaction, however the consumer does not enter their pin number because it is usually an Electronic Funds Transfer or Ecommerce transaction.
See also How can I get pinless debit?