You are not going to carry the stamp - you are already carrying your mobile device. The stamp is placed with the centralized person or system with whom you want to interact (e.g. at the coffee shop's point of sale or on the outside of the door you want to unlock). You stamp your mobile device to authenticate that you are interacting with that person or system in a specific way (e.g. the coffee shop cashier only stamps the loyalty app on your phone if you have made a purchase - thus purchase-gating that transaction). Does that make sense?

So I still have to type in a pin to prove its me? If I've already got my smartphone it would seem either NFC or Protean would be simpler.