Functionally, the "backup" key (which looks like a credit card in Tesla's) is identical to the backup mechanism to a traditional keyless entry fob. If you've ever had a keyless entry fob go dead because the battery runs out, you start the car by holding the key fob over a special location (usually either the start button or on the steering column where a key would be). Same concept, but the backup key for a Tesla doesn't have the integrated key for normal keyless functionality. That's what the cellphone all is for.