A person enters a public place e.g., bar, cafe, train station, etc., with a large bag. The bag has the bottom of it cut out. This person then puts the bag over someone's purse or any other item and a mechanism inside the bag grabs hold of the item. The thief then simply walks away as nothing has happened. This theft can be hard to spot as it is done quickly and silently.
Booster bag also refers to a shopping bag that has been lined with tin foil to prevent RFID Anti-Theft devices from activating when you walk out the door.