RFID-blocking wallets have card sleeves (or sometimes entire wallets) made from materials that don't let radio waves through. That way, the chip won't power up, and even if it did, its signal wouldn't get through the wallet. The bottom line is that you can't read the RFID card through the wallet.

They encase your credit cards in a material that interferes with radio waves. If the wallet is properly constructed as a Faraday cage, it will block all electromagnetic fields and prevent communication between your cards and RFID scanners.